Filed Pursuant to Rule 424(b)(3)
Registration No. 333-219137

PROSPECTUS SUPPLEMENT

To Prospectus dated December 15, 2017

7,234,838 Shares

Atkore International Group Inc.

Common Stock

All of the shares of common stock of Atkore International Group Inc. (“Atkore” or the “Company”) are being sold by the selling stockholders identified in this prospectus supplement. We will not receive any of the proceeds from the sale of the shares being sold by the selling stockholders.

The common stock of Atkore is listed on the New York Stock Exchange (“NYSE”) under the symbol “ATKR”. The last reported sale price of the common stock on May 15, 2018 was $20.59 per share.

Investing in our common stock involves risks. See “Risk Factors” on page S-7 to read about factors you should consider before buying shares of our common stock.

The underwriter has agreed to purchase the shares of our common stock from the selling stockholders at a price of $19.91 per share, which will result in $144,045,624.58 of proceeds to the selling stockholders before expenses. The underwriter proposes to offer the shares of common stock from time to time for sale in one or more transactions on the NYSE, in the over-the-counter market, through negotiated transactions or otherwise at market prices prevailing at the time of sale, at prices related to prevailing market prices or at negotiated prices. See “Underwriting” for additional information regarding underwriting compensation

Neither the U.S. Securities and Exchange Commission (“SEC”) nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus supplement. Any representation to the contrary is a criminal offense.

The underwriter expects to deliver the shares to purchasers on or about May 21, 2018.

Credit Suisse

The date of this prospectus supplement is May 16, 2018.

TABLE OF CONTENTS

Prospectus Supplement

	Page
About This Prospectus Supplement		S-1
Special Note Regarding Forward-Looking Statements and Information		S-2
Prospectus Supplement Summary		S-5
Risk Factors		S-7
Use of Proceeds		S-13
Market Price of Our Common Stock		S-14
Dividend Policy		S-15
U.S. Federal Income Tax Considerations for Non-U.S. Holders		S-16
Selling Stockholders		S-20
Underwriting		S-21
Legal Matters		S-27
Experts		S-28
Where You Can Find More Information		S-28
Incorporation by Reference		S-28

Prospectus

	Page
About This Prospectus		1
Special Note Regarding Forward-Looking Statements and Information		2
Our Company		5
Risk Factors		6
Use of Proceeds		6
Description of Capital Stock		6
Selling Stockholder		12
Plan of Distribution		13
U.S. Federal Income Tax Considerations for Non-U.S. Holders		16
Legal Matters		20
Experts		20
Where You Can Find More Information		20
Incorporation of Certain Information by Reference		21

Neither we, the selling stockholders, nor the underwriter has authorized anyone to provide you with different information or to make any representations other than those contained or incorporated by reference into this prospectus supplement, the accompanying prospectus or in any free writing prospectuses we have prepared. If anyone provides you with different or inconsistent information, you should not rely on it. This prospectus supplement and the accompanying prospectus do not constitute an offer to sell, or a solicitation of an offer to purchase, the securities offered by this prospectus supplement and the accompanying prospectus in any jurisdiction in which it is unlawful to make such offer or solicitation. You should assume that the information contained in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference is accurate only as of the date such information is presented. Our business, financial condition and results of operations may have changed since the date such information was presented.

For investors outside the United States: Neither we, the selling stockholders nor the underwriter has done anything that would permit this offering or possession or distribution of this prospectus supplement and the accompanying prospectus in any jurisdiction where action for that purpose is required, other than in the United States. You are required to inform yourselves about and to observe any restrictions relating to this offering and the distribution of this prospectus supplement and the accompanying prospectus outside of the United States.

ABOUT THIS PROSPECTUS SUPPLEMENT

This document has two parts, a prospectus supplement and an accompanying prospectus dated December 15, 2017. This prospectus supplement and the accompanying prospectus are part of a registration statement on Form S-3 that we filed with the SEC utilizing a “shelf” registration process. Under this shelf registration process, the selling stockholders named in a prospectus supplement may, from time to time, offer and sell our common stock in one or more offerings or resales.

The accompanying prospectus provides you with a general description of our common stock, which the selling stockholders may offer pursuant to this prospectus supplement. This prospectus supplement, which describes certain matters relating to us and the specific terms of this offering of shares of our common stock, adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference herein and therein. Any statement that we make in the accompanying prospectus will be deemed modified or superseded by any inconsistent statement made by us in this prospectus supplement.

The rules of the SEC allow us to incorporate by reference information into this prospectus supplement. This information incorporated by reference is considered to be a part of this prospectus supplement, and information that we file later with the SEC, to the extent incorporated by reference, will automatically update and supersede this information. See “Incorporation by Reference.” You should read both this prospectus supplement and the accompanying prospectus together with the additional information incorporated by reference herein and therein, including all documents described under the headings “Incorporation by Reference” and “Where You Can Find Additional Information” in this prospectus supplement before investing in our common stock.

Unless the context otherwise requires, references in this prospectus to “Atkore”, the “Company”, “ATKR”, “we”, “us” and “our” mean Atkore International Group Inc.

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS AND INFORMATION

This prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein contain forward-looking statements and cautionary statements within the meaning of the Private Securities Litigation Reform Act of 1995 that are based on management’s beliefs and assumptions and information currently available to management. Some of the forward-looking statements can be identified by the use of forward-looking terms such as “believes,” “expects,” “may,” “will,” “shall,” “should,” “would,” “could,” “seeks,” “aims,” “projects,” “is optimistic,” “intends,” “plans,” “estimates,” “anticipates” or other comparable terms. Forward-looking statements include, without limitation, all matters that are not historical facts. They appear in a number of places throughout this prospectus supplement, the accompanying prospectus or in the documents incorporated by reference herein or therein and include, without limitation, statements regarding our intentions, beliefs, assumptions or current expectations concerning, among other things, financial position; results of operations; cash flows; prospects; growth strategies or expectations; timing and amount of share repurchases; customer retention; the outcome (by judgment or settlement) and costs of legal, administrative or regulatory proceedings, investigations or inspections, including, without limitation, collective, representative or class action litigation; and the impact of prevailing economic conditions.

Forward-looking statements are subject to known and unknown risks and uncertainties, many of which may be beyond our control. We caution you that forward-looking statements are not guarantees of future performance or outcomes and that actual performance and outcomes, including, without limitation, our actual results of operations, financial condition and liquidity, and the development of the market in which we operate, may differ materially from those made in or suggested by the forward-looking statements contained in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein. In addition, even if our results of operations, financial condition and cash flows, and the development of the market in which we operate, are consistent with the forward-looking statements contained in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein, those results or developments may not be indicative of results or developments in subsequent periods. A number of important factors, including, without limitation, the risks and uncertainties discussed or referenced under the caption “Risk Factors” in this prospectus supplement, the accompanying prospectus and those described from time to time in our other filings with the SEC, could cause actual results and outcomes to differ materially from those reflected in the forward-looking statements. Additional factors that could cause actual results and outcomes to differ from those reflected in forward-looking statements include, without limitation:

• declines in, and uncertainty regarding, the general business and economic conditions in the U.S. and international markets in which we operate;

• weakness or another downturn in the U.S. non-residential construction industry;

• changes in prices of raw materials;

• pricing pressure, reduced profitability, or loss of market share due to intense competition;

• availability and cost of third-party freight carriers and energy;

• high levels of imports of products similar to those manufactured by us;

• changes in federal, state, local and international governmental regulations and trade policies;

• adverse weather conditions;

• failure to generate sufficient cash flow from operations or to raise sufficient funds in the capital markets to satisfy existing obligations and support the development of our business;

• increased costs relating to future capital and operating expenditures to maintain compliance with environmental, health and safety laws;

• reduced spending by, deterioration in the financial condition of, or other adverse developments with respect to, one or more of our top customers;

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• increases in our working capital needs, which are substantial and fluctuate based on economic activity and the market prices for our main raw materials, including as a result of failure to collect, or delays in the collection of, cash from the sale of manufactured products;

• work stoppage or other interruptions of production at our facilities as a result of disputes under existing collective bargaining agreements with labor unions or, in connection with negotiations of new collective bargaining agreements, as a result of supplier financial distress, or for other reasons;

• challenges attracting and retaining key personnel or high-quality employees;

• changes in our financial obligations relating to pension plans that we maintain in the United States;

• reduced production or distribution capacity due to interruptions in the operations of our facilities or those of our key suppliers;

• loss of a substantial number of our third-party agents or distributors or a dramatic deviation from the amount of sales they generate;

• security threats, attacks, or other disruptions to our information systems, or failure to comply with complex network security, data privacy and other legal obligations or the failure to protect sensitive information;

• possible impairment of goodwill or other long-lived assets as a result of future triggering events, such as declines in our cash flow projections or customer demand;

• safety and labor risks associated with the manufacture and in the testing of our products;

• product liability, construction defect and warranty claims and litigation relating to our various products, as well as government inquiries and investigations, and consumer, employment, tort and other legal proceedings;

• our ability to protect our intellectual property and other material proprietary rights;

• risks inherent in doing business internationally;

• our inability to introduce new products effectively or implement our innovation strategies;

• the inability of our customers to pay off the credit lines extended to them by us in a timely manner and the negative impact on customer relations resulting from our collections efforts with respect to non- paying or slow-paying customers;

• our inability to continue importing raw materials, component parts and/or finished goods;

• changes as a result of comprehensive tax reform;

• the incurrence of liabilities and the issuance of additional debt or equity in connection with acquisitions, joint ventures or divestitures and the failure of indemnification provisions in our acquisition agreements to fully protect us from unexpected liabilities;

• failure to manage acquisitions successfully, including identifying, evaluating, and valuing acquisition targets and integrating acquired companies, businesses or assets;

• the incurrence of liabilities in connection with violations of the FCPA and similar foreign anti- corruption laws;

• the incurrence of additional expenses, increase in complexity of our supply chain and potential damage to our reputation with customers resulting from regulations related to “conflict minerals”;

• disruptions or impediments to the receipt of sufficient raw materials resulting from various anti- terrorism security measures;

• restrictions contained in our debt agreements;

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• failure to generate cash sufficient to pay the principal of, interest on, or other amounts due on our debt; and

• other risks and factors included under “Risk Factors” and elsewhere in this prospectus supplement.

You should read this prospectus supplement and the accompanying prospectus, including the uncertainties and factors discussed under “Risk Factors” and the documents incorporated by reference herein and therein completely and with the understanding that actual future results may be materially different from expectations. All forward-looking statements attributable to us or persons acting on our behalf that are made in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein are qualified in their entirety by these cautionary statements. These forward-looking statements are made only as of the date presented, and we do not undertake any obligation, other than as may be required by law, to update or revise any forward-looking or cautionary statements to reflect changes in assumptions, the occurrence of events, unanticipated or otherwise, and changes in future operating results over time or otherwise.

Comparisons of results for current and any prior periods are not intended to express any future trends, or indications of future performance, unless expressed as such, and should only be viewed as historical data.

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PROSPECTUS SUPPLEMENT SUMMARY

This summary highlights important features of this offering and the information included or incorporated by reference in this prospectus supplement and the accompanying prospectus. This summary does not contain all of the information that you should consider before making any investment decision. You should read carefully the entire prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein, especially the risks of investing in our common stock discussed under “Risk Factors.”

Our Company

We are a leading manufacturer of Electrical Raceway products primarily for the non-residential construction and renovation markets and Mechanical Products & Solutions for the construction and industrial markets. Electrical Raceway products form the critical infrastructure that enables the deployment, isolation and protection of a structure’s electrical circuitry from the original power source to the final outlet. Mechanical Products & Solutions frame, support and secure component parts in a broad range of structures, equipment and systems in electrical, industrial and construction applications. The quality of our products, the strength of our brands and our scale and national presence provide what we believe to be a unique set of competitive advantages that positions us for profitable growth.

We manufacture a broad range of end-to-end integrated products and solutions that are critical to our customers’ businesses. Our broad product offering enables us to bundle and co-load a wide range of products, which simplifies the ordering and delivery processes and streamlines logistics, reducing costs for us and our customers. We primarily serve electrical contractors and original equipment manufacturers, both directly and through our established core customer base of electrical and industrial distributors. Our operational footprint, together with our national distribution network, provides important proximity to our customers and enables efficient and reliable delivery of our products. Our scale creates meaningful purchasing power with key suppliers and enables us to leverage common manufacturing technology and processes across our business.

Our principal executive offices are located at 16100 South Lathrop Avenue, Harvey, Illinois 60426, and our telephone number at that address is (708) 339-1610.

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THE OFFERING

Common stock offered by the selling stockholders 7,234,838 shares

Common stock outstanding after the offering 46,577,795 shares

NYSE symbol “ATKR”

Use of proceeds We will not receive any proceeds from the sale of our common stock by the selling stockholders.

Risk factors See “Risk Factors” and other information included in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein for a discussion of factors you should carefully consider before deciding whether to invest in shares of our common stock.

Dividend policy We do not currently anticipate paying dividends on our common stock for the foreseeable future. Any future determination to pay dividends on our common stock will be subject to the discretion of our board of directors and depend upon various factors. See “Dividend Policy.”

The number of shares of common stock that will be outstanding after this offering is based on 46,577,795 shares outstanding as of April 27, 2018, and excludes:

• 4,544,025 shares of common stock issuable upon exercise of options outstanding as of April 27, 2018 at a weighted average exercise price of $8.94 per share;

• 576,661 shares of our common stock subject to outstanding restricted stock units (“RSUs”) as of April 27, 2018;

• 381,286 shares of our common stock subject to outstanding performance share units (“PSUs”) as of April 27, 2018; and

• 2,281,792 shares of our common stock reserved for issuance for future awards under the Atkore International Group Inc. 2016 Omnibus Incentive Plan as of April 27, 2018.

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RISK FACTORS

Investing in our common stock involves a high degree of risk. You should consider and read carefully all of the risks and uncertainties described below, as well as other information contained or incorporated by reference in this prospectus supplement and any accompanying prospectus. In particular, we urge you to consider carefully the risks and uncertainties discussed in “Part I—Item 1A—Risk Factors” of our Annual Report on Form 10-K for the year ended September 30, 2017, as such risk factors are updated by our annual, quarterly and current reports that we file with the SEC and that are incorporated by reference in this prospectus supplement and the accompanying prospectus. The risks described below are not the only ones facing us. The occurrence of any of, or a combination of, the following risks or additional risks and uncertainties not presently known to us or that we currently believe to be immaterial could materially and adversely affect our business, financial position, results of operations or cash flows. In any such case, the trading price of our common stock could decline, and you may lose all or part of your investment. This prospectus supplement also contains forward-looking statements and estimates that involve risks and uncertainties. Our actual results could differ materially from those anticipated in the forward-looking statements as a result of specific factors, including the risks and uncertainties described below.

Risks Related to Our Common Stock and This Offering

Atkore is a holding company with no operations of its own, and it depends on its subsidiaries for cash to fund all of its operations and expenses, including to make future dividend payments, if any.

Our operations are conducted entirely through our subsidiaries, and our ability to generate cash to fund our operations and expenses, to pay dividends or to meet debt service obligations is highly dependent on the earnings and the receipt of funds from our subsidiaries through dividends or intercompany loans. Deterioration in the financial condition, earnings or cash flow of Atkore International, Inc. (“AII”) and its subsidiaries for any reason could limit or impair their ability to pay such distributions. Additionally, to the extent our subsidiaries are restricted from making such distributions under applicable law or regulation or under the terms of our financing arrangements, or are otherwise unable to provide funds to the extent of our needs, there could be a material adverse effect on our business, financial condition, results of operations or cash flows.

For example, the agreements governing AII’s credit facilities (“Credit Facilities”) significantly restrict the ability of our subsidiaries to pay dividends, make loans or otherwise transfer assets to us. Furthermore, our subsidiaries are permitted under the terms of the Credit Facilities to incur additional indebtedness that may restrict or prohibit the making of distributions, the payment of dividends or the making of loans by such subsidiaries to us.

The market price of our common stock may be volatile and could decline after this offering.

Volatility in the market price of our common stock may prevent you from being able to sell your shares at or above the price you paid for your shares. The market price of our common stock may fluctuate significantly. Among the factors that could affect our stock price are:

• industry or general market conditions;

• domestic and international economic factors unrelated to our performance;

• changes in our customers’ preferences;

• new regulatory pronouncements and changes in regulatory guidelines;

• lawsuits, enforcement actions and other claims by third parties or governmental authorities;

• actual or anticipated fluctuations in our quarterly operating results;

• changes in securities analysts’ estimates of our financial performance or lack of research coverage and reports by industry analysts;

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• action by institutional stockholders or other large stockholders, including future sales of our common stock;

• failure to meet any guidance given by us or any change in any guidance given by us, or changes by us in our guidance practices;

• announcements by us of significant impairment charges;

• speculation in the press or investment community;

• investor perception of us and our industry;

• changes in market valuations or earnings of similar companies;

• announcements by us or our competitors of significant contracts, acquisitions, dispositions or strategic partnerships;

• war, terrorist acts and epidemic disease;

• any future sales of our common stock or other securities;

• additions or departures of key personnel; and

• misconduct or other improper actions of our employees.

In particular, we cannot assure you that you will be able to resell your shares at or above the public offering price. Stock markets have experienced extreme volatility in recent years that has been unrelated to the operating performance of particular companies. These broad market fluctuations may adversely affect the trading price of our common stock. In the past, following periods of volatility in the market price of a company’s securities, class action litigation has often been instituted against the affected company. Any litigation of this type brought against us could result in substantial costs and a diversion of our management’s attention and resources, which could materially and adversely affect our business, financial position, results of operations or cash flows.

Future sales of shares by us or our existing stockholders could cause our stock price to decline.

Sales of substantial amounts of our common stock in the public market following this offering, or the perception that these sales could occur, could cause the market price of our common stock to decline. These sales, or the possibility that these sales may occur, also might make it more difficult for us to sell equity securities in the future at a time and at a price that we deem appropriate.

As of April 27, 2018, we had 46,577,795 outstanding shares of common stock. Of these shares, all of the 31,550,000 shares sold in our IPO and secondary offerings by CD&R Allied Holdings, L.P., or the “CD&R Investor”, are, and the 7,234,838 shares to be sold in this offering will be, immediately tradable without restriction under the Securities Act of 1933, as amended, or the “Securities Act,” except for any shares held by “affiliates,” as that term is defined in Rule 144 under the Securities Act, or “Rule 144.”

In June 2016, we filed a registration statement on Form S-8 under the Securities Act to register the shares of common stock to be issued under our equity compensation plans and, as a result, all shares of common stock issued upon exercise of (i) stock options granted under these plans and (ii) other equity based awards granted under the Atkore International Group Inc. 2016 Omnibus Incentive Plan, or the “Omnibus Incentive Plan”, were also freely tradable under the Securities Act, subject to the terms of the lock-up agreements, unless purchased by our affiliates. As of April 27, 2018, there were stock options outstanding to purchase a total of 4,544,025 shares of our common stock. Additionally, 576,661 shares of our common stock are issuable pursuant to RSUs and 381,286 shares of our common stock are issuable pursuant to PSUs. As of April 27, 2018, 2,281,792 shares of our common stock were reserved for issuance for future awards under the Omnibus Incentive Plan.

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In connection with this offering, we and our executive officers and directors will sign lock-up agreements under which, subject to certain exceptions, we and they have agreed not to sell, transfer or dispose of or hedge, directly or indirectly, any shares of our common stock or any securities convertible into or exercisable or exchangeable for shares of our common stock for a period of 30 days after the date of this prospectus supplement except with the prior written consent of the underwriter. See “Underwriting.” Following the expiration of this 30-day lock-up period, such shares of our common stock will be eligible for future sale, subject to the applicable volume, manner of sale, holding period and other limitations of Rule 144 or pursuant to an exemption from registration under Rule 701. The market price of our common stock could decline if the holders of those shares sell them or are perceived by the market as intending to sell them.

In the future, we may issue additional shares of common stock or other equity or debt securities convertible into or exercisable or exchangeable for shares of our common stock in connection with a financing, strategic investment, litigation settlement or employee arrangement or otherwise. Any of these issuances could result in substantial dilution to our existing stockholders and could cause the trading price of our common stock to decline.

The timing and amount of the Company’s share repurchases are subject to a number of uncertainties.

In August 2017, we announced that our board of directors had approved a share repurchase program for the repurchase of up to an aggregate amount of $75 million of our common stock. Share repurchases under the program are funded with cash on hand. The amount and timing of share repurchases will be based on a variety of factors. Important factors that could cause us to limit, suspend or delay its share repurchases include unfavorable trading market conditions, the price of the our common stock, the nature of other investment opportunities presented to us from time to time, the ability to obtain financing at attractive rates and the availability of U.S. cash. The share repurchase program does not obligate us to acquire any particular amount of common stock, and it may be terminated at any time at our discretion.

If securities or industry analysts do not publish research or publish misleading or unfavorable research about our business, our stock price and trading volume could decline.

The trading market for our common stock depends in part on the research and reports that securities or industry analysts publish about us or our business. If one or more of the analysts that covers our common stock downgrades our stock or publishes misleading or unfavorable research about our business, our stock price would likely decline. If one or more of the analysts ceases coverage of our common stock or fails to publish reports on us regularly, demand for our common stock could decrease, which could cause our common stock price or trading volume to decline.

Future offerings of debt or equity securities which would rank senior to our common stock may adversely affect the market price of our common stock.

If, in the future, we decide to issue debt or equity securities that rank senior to our common stock, it is likely that such securities will be governed by an indenture or other instrument containing covenants restricting our operating flexibility. Additionally, any convertible or exchangeable securities that we issue in the future may have rights, preferences and privileges more favorable than those of our common stock and may result in dilution to owners of our common stock. We and, indirectly, our stockholders, will bear the cost of issuing and servicing such securities. Because our decision to issue debt or equity securities in any future offering will depend on market conditions and other factors beyond our control, we cannot predict or estimate the amount, timing or nature of our future offerings. Thus, holders of our common stock will bear the risk of our future offerings reducing the market price of our common stock and diluting the value of their stock holdings in us.

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Fulfilling our obligations incident to being a public company, including compliance with the requirements of the Exchange Act, and related rules under the Sarbanes-Oxley Act of 2002, or the “Sarbanes-Oxley Act,” and the Dodd-Frank Act, will be expensive and time-consuming, and any delays or difficulties in satisfying these obligations could have a material adverse effect on our future results of operations and our stock price.

Our IPO was completed in June 2016. As a public company, we are subject to the reporting, accounting and corporate governance requirements of the NYSE, the Exchange Act, the Sarbanes-Oxley Act and the Dodd-Frank Act that apply to issuers of listed equity, which impose certain significant compliance requirements, costs and obligations upon us. The changes necessitated by being a publicly listed company require a significant commitment of additional resources and management oversight which increase our operating costs. Further, to comply with the requirements of being a public company, we may need to undertake various actions, such as implementing new internal controls and procedures and hiring additional accounting or internal audit staff. In addition, we may identify control deficiencies which could result in a material weakness or significant deficiency. In the past, we have identified material weaknesses, all of which have since been remediated. We did not identify any material weaknesses for fiscal 2017.

The expenses associated with being a public company include increases in auditing, accounting and legal fees and expenses, investor relations expenses, increased directors’ fees and director and officer liability insurance costs, registrar and transfer agent fees and listing fees, as well as other expenses. As a public company, we are required, among other things, to define and expand the roles and the duties of our board of directors and its committees and institute more comprehensive compliance and investor relations functions. Failure to comply with Sarbanes-Oxley Act or Dodd-Frank Act could potentially subject us to sanctions or investigations by the SEC, the NYSE or other regulatory authorities.

Anti-takeover provisions in our amended and restated certificate of incorporation and amended and restated by-laws could discourage, delay or prevent a change of control of our company and may affect the trading price of our common stock.

Our amended and restated certificate of incorporation and our second amended and restated by-laws, (“amended and restated by-laws”) include a number of provisions that may discourage, delay or prevent a change in our management or control over us that stockholders may consider favorable. For example, our amended and restated certificate of incorporation and amended and restated by-laws collectively:

• authorize the issuance of “blank check” preferred stock that could be issued by our board of directors to thwart a takeover attempt;

• provide for a classified board of directors, which divides our board of directors into three classes, with members of each class serving staggered three-year terms, which prevents stockholders from electing an entirely new board of directors at an annual meeting;

• limit the ability of stockholders to remove directors;

• provide that vacancies on our board of directors, including vacancies resulting from an enlargement of our board of directors, may be filled only by a majority vote of directors then in office;

• prohibit stockholders from calling special meetings of stockholders;

• prohibit stockholder action by written consent, thereby requiring all actions to be taken at a meeting of the stockholders;

• establish advance notice requirements for nominations of candidates for election as directors or to bring other business before an annual meeting of our stockholders; and

• require the approval of holders of at least 66 ²⁄₃% of the outstanding shares of our common stock to amend our amended and restated by-laws and certain provisions of our amended and restated certificate of incorporation.

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These provisions may prevent our stockholders from receiving the benefit from any premium to the market price of our common stock offered by a bidder in a takeover context. Even in the absence of a takeover attempt, the existence of these provisions may adversely affect the prevailing market price of our common stock if the provisions are viewed as discouraging takeover attempts in the future. See “Description of Capital Stock—Anti-Takeover Effects of our Certificate of Incorporation and By-Laws—Classified Board of Directors” in the accompanying prospectus.

Our amended and restated certificate of incorporation and amended and restated by-laws may also make it difficult for stockholders to replace or remove our management. Furthermore, the existence of the foregoing provisions could limit the price that investors might be willing to pay in the future for shares of our common stock. These provisions may facilitate management entrenchment that may delay, deter, render more difficult or prevent a change in our control, which may not be in the best interests of our stockholders.

We could be the subject of securities class action litigation due to future stock price volatility, which could divert management’s attention and materially and adversely affect our business, financial position, results of operations or cash flows.

The stock market in general, and market prices for the securities of companies like ours in particular, have from time to time experienced volatility that often has been unrelated to the operating performance of the underlying companies. These broad market and industry fluctuations may adversely affect the market price of our common stock, regardless of our operating performance. In certain situations in which the market price of a stock has been volatile, holders of that stock have instituted securities class action litigation against the company that issued the stock. If any of our stockholders were to bring a similar lawsuit against us, the defense and disposition of the lawsuit could be costly and divert the time and attention of our management and could materially and adversely affect our business, financial position, results of operations or cash flows.

We do not currently intend to pay dividends on our common stock for the foreseeable future and, consequently, your ability to achieve a return on your investment depends on appreciation in the price of our common stock.

We do not currently intend to declare and pay dividends on our common stock for the foreseeable future. We currently intend to use our future earnings, if any, to repay debt, to fund our growth, to develop our business, for working capital needs and for general corporate purposes. Therefore, you are not likely to receive any dividends on your common stock for the foreseeable future, and the success of an investment in shares of our common stock depends upon any future appreciation in their value. There is no guarantee that shares of our common stock will appreciate in value or even maintain the price at which our stockholders have purchased their shares. Payments of dividends, if any, are at the sole discretion of our board of directors after taking into account various factors, including general and economic conditions, our financial condition and operating results, our available cash and current and anticipated cash needs, capital requirements, contractual, legal, tax and regulatory restrictions and implications of the payment of dividends by us to our stockholders or by our subsidiaries (including AII) to us, and such other factors as our board of directors may deem relevant. In addition, our operations are conducted almost entirely through our subsidiaries. As such, to the extent that we determine in the future to pay dividends on our common stock, none of our subsidiaries will be obligated to make funds available to us for the payment of dividends. Further, the agreements governing our Credit Facilities significantly restrict the ability of our subsidiaries to pay dividends or otherwise transfer assets to us. In addition, Delaware law imposes additional requirements that may restrict our ability to pay dividends to holders of our common stock.

Our amended and restated certificate of incorporation includes provisions limiting the personal liability of our directors for breaches of fiduciary duty under the DGCL.

Our amended and restated certificate of incorporation contains provisions permitted under the action asserting a claim arising under the General Corporation Law of the State of Delaware (“DGCL”) relating to the

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liability of directors. These provisions eliminate a director’s personal liability to the fullest extent permitted by the DGCL for monetary damages resulting from a breach of fiduciary duty, except in circumstances involving:

• any breach of the director’s duty of loyalty;

• acts or omissions not in good faith or which involve intentional misconduct or a knowing violation of the law;

• under Section 174 of the DGCL (unlawful dividends); and

• any transaction from which the director derives an improper personal benefit.

The principal effect of the limitation on liability provision is that a stockholder will be unable to prosecute an action for monetary damages against a director unless the stockholder can demonstrate a basis for liability for which indemnification is not available under the DGCL. These provisions, however, should not limit or eliminate our rights or any stockholder’s rights to seek non-monetary relief, such as an injunction or rescission, in the event of a breach of a director’s fiduciary duty. These provisions do not alter a director’s liability under federal securities laws. The inclusion of this provision in our amended and restated certificate of incorporation may discourage or deter stockholders or management from bringing a lawsuit against directors for a breach of their fiduciary duties, even though such an action, if successful, might otherwise have benefited us and our stockholders.

Our amended and restated certificate of incorporation designates the Court of Chancery of the State of Delaware as the sole and exclusive forum for certain litigation that may be initiated by our stockholders, which could limit our stockholders’ ability to obtain a favorable judicial forum for disputes with us or our directors, officers or stockholders.

Our amended and restated certificate of incorporation provides that, unless we consent in writing to the selection of an alternative forum, the Court of Chancery of the State of Delaware is, to the fullest extent permitted by law, the sole and exclusive forum for (i) any derivative action or proceeding brought on our behalf, (ii) any action asserting a claim of breach of a fiduciary duty owed to us or our stockholders by any of our directors, officers, other employees, agents or stockholders, (iii) any action asserting a claim arising out of or under the DGCL, or as to which the DGCL confers jurisdiction on the Court of Chancery of the State of Delaware (including, without limitation, any action asserting a claim arising out of or pursuant to our amended and restated certificate of incorporation or our amended and restated by-laws) or (iv) any action asserting a claim that is governed by the internal affairs doctrine. By becoming a stockholder in our company, you will be deemed to have notice of and have consented to the provisions of our amended and restated certificate of incorporation related to choice of forum. The choice of forum provision in our amended and restated certificate of incorporation may limit our stockholders’ ability to obtain a favorable judicial forum for disputes with us or any of our directors, officers, other employees, agents or stockholders, which may discourage lawsuits with respect to such claims. Alternatively, if a court were to find the choice of forum provision contained in our amended and restated certificate of incorporation to be inapplicable or unenforceable in an action, we may incur additional costs associated with resolving such action in other jurisdictions, which could materially and adversely affect our business, financial position, results of operations or cash flows.

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USE OF PROCEEDS

The selling stockholders will receive all of the net proceeds from the sale of shares of our common stock offered pursuant to this prospectus supplement. Accordingly, we will not receive any proceeds from the sale of the shares being sold in this offering. The selling stockholders will bear any underwriting commissions and discounts attributable to its sale of our common stock, and we will bear the remaining expenses.

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MARKET PRICE OF OUR COMMON STOCK

Our common stock has been listed on the NYSE under the symbol “ATKR” since June 10, 2016. Prior to that time, there was no public market for our common stock. The following table sets forth for the periods indicated the high and low sales prices per share of our common stock as reported on the NYSE:

	High		Low
2016:
Third quarter (1)	$	16.85	$	15.56
Fourth quarter	$	19.17	$	14.17
2017:
First quarter	$	24.34	$	18.00
Second quarter	$	27.30	$	22.96
Third quarter	$	26.80	$	20.64
Fourth quarter	$	23.43	$	15.05
2018:
First quarter	$	22.50	$	18.56
Second quarter	$	24.48	$	19.25
Third quarter (through May 15, 2018)		$21.07		$17.60

(1) Represents the period from June 10, 2016, the date of our initial listing on the NYSE, through June 24, 2016, the end of our 2016 third fiscal quarter.

A recent reported closing price for our common stock is set forth on the cover page of this prospectus supplement. As of April 27, 2018, there were 2 stockholders of record of our common stock. The actual number of stockholders is greater than this number of record holders, and includes stockholders who are beneficial owners but whose shares are held in street name by brokers and other nominees.

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DIVIDEND POLICY

We do not currently intend to declare or pay dividends on our common stock for the foreseeable future. We currently intend to use our future earnings, if any, to repay debt, to fund our growth, to develop our business, for working capital needs and general corporate purposes. Our ability to pay dividends to holders of our common stock is significantly limited as a practical matter by the Credit Facilities insofar as we may seek to pay dividends out of funds made available to us by AII or its subsidiaries, because AII’s debt instruments directly or indirectly restrict AII’s ability to pay dividends or make loans to us. Any future determination to pay dividends on our common stock will be subject to the discretion of our board of directors and depend upon various factors, including our results of operations, financial condition, liquidity requirements, capital requirements, level of indebtedness, contractual restrictions with respect to payment of dividends, restrictions imposed by Delaware law, general business conditions and other factors that our board of directors may deem relevant.

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U.S. FEDERAL INCOME TAX CONSIDERATIONS FOR NON-U.S. HOLDERS

The following is a discussion of certain U.S. federal income tax considerations relating to the purchase, ownership and disposition of our common stock by Non-U.S. Holders (as defined below) that purchase our common stock pursuant to this prospectus supplement and the accompanying prospectus and hold such common stock as a capital asset. This discussion is based on the U.S. Internal Revenue of 1986, as amended (the “Code”), U.S. Treasury regulations promulgated or proposed thereunder, and administrative and judicial interpretations thereof, all as in effect on the date hereof and all of which are subject to change, possibly with retroactive effect, or to different interpretation. This discussion does not address all of the U.S. federal income tax considerations that may be relevant to specific Non-U.S. Holders in light of their particular circumstances or to Non-U.S. Holders subject to special treatment under U.S. federal income tax law (such as banks, insurance companies, dealers in securities or other Non-U.S. Holders that generally mark their securities to market for U.S. federal income tax purposes, foreign governments, international organizations, tax-exempt entities, certain former citizens or residents of the United States, or Non-U.S. Holders that hold our common stock as part of a straddle, hedge, conversion or other integrated transaction). This discussion does not address any U.S. state or local or non-U.S. tax considerations or any U.S. federal gift or alternative minimum tax considerations.

As used in this discussion, the term “Non-U.S. Holder” means a beneficial owner of our common stock that, for U.S. federal income tax purposes, is:

• an individual who is neither a citizen nor a resident of the United States;

• a corporation that is not created or organized in or under the laws of the United States, any state thereof, or the District of Columbia;

• an estate that is not subject to U.S. federal income tax on income from non-U.S. sources that is not effectively connected with the conduct of a trade or business in the United States; or

• a trust unless (i) a court within the United States is able to exercise primary supervision over its administration and one or more U.S. persons have the authority to control all of its substantial decisions or (ii) it has in effect a valid election under applicable U.S. Treasury regulations to be treated as a U.S. person.

If an entity treated as a partnership for U.S. federal income tax purposes invests in our common stock, the U.S. federal income tax considerations relating to such investment will depend in part upon the status and activities of such entity and the particular partner. Any such entity should consult its own tax advisor regarding the U.S. federal income tax considerations applicable to it and its partners relating to the purchase, ownership and disposition of our common stock.

PERSONS CONSIDERING AN INVESTMENT IN OUR COMMON STOCK SHOULD CONSULT THEIR OWN TAX ADVISORS REGARDING THE U.S. FEDERAL, STATE AND LOCAL AND NON-U.S. INCOME, ESTATE AND OTHER TAX CONSIDERATIONS RELATING TO THE PURCHASE, OWNERSHIP AND DISPOSITION OF OUR COMMON STOCK IN LIGHT OF THEIR PARTICULAR CIRCUMSTANCES.

Distributions on Common Stock

If we make a distribution of cash or other property (other than certain pro rata distributions of our common stock or rights to acquire our common stock) with respect to a share of our common stock, the distribution generally will be treated as a dividend to the extent it is paid from our current or accumulated earnings and profits (as determined under U.S. federal income tax principles). If the amount of such distribution exceeds our current and accumulated earnings and profits, such excess generally will be treated first as a tax-free return of

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capital to the extent of the Non-U.S. Holder’s adjusted tax basis in such share of our common stock, and then as capital gain (which will be treated in the manner described below under “—Sale, Exchange or Other Disposition of Common Stock”). Distributions treated as dividends on our common stock that are paid to or for the account of a Non-U.S. Holder generally will be subject to U.S. federal withholding tax at a rate of 30%, or at a lower rate if provided by an applicable tax treaty and the Non-U.S. Holder provides the documentation (generally, Internal Revenue Service (“IRS”) Form W-8BEN or W-8BEN-E) required to claim benefits under such tax treaty to the applicable withholding agent. Even if our current or accumulated earnings and profits are less than the amount of the distribution, the applicable withholding agent may elect to treat the entire distribution as a dividend for U.S. federal withholding tax purposes. Each Non-U.S. Holder should consult its own tax advisor regarding U.S. federal withholding tax on distributions, including such Non-U.S. Holder’s eligibility for a lower rate and the availability of a refund of any excess U.S. federal tax withheld.

If, however, a dividend is effectively connected with the conduct of a trade or business in the United States by a Non-U.S. Holder, such dividend generally will not be subject to the 30% U.S. federal withholding tax if such Non-U.S. Holder provides the appropriate documentation (generally, IRS Form W-8ECI) to the applicable withholding agent. Instead, such Non-U.S. Holder generally will be subject to U.S. federal income tax on such dividend in substantially the same manner as a U.S. person (except as provided by an applicable tax treaty). In addition, a Non-U.S. Holder that is treated as a corporation for U.S. federal income tax purposes may be subject to a branch profits tax at a rate of 30% (or a lower rate if provided by an applicable tax treaty) on its effectively connected income for the taxable year, subject to certain adjustments.

The foregoing discussion is subject to the discussion below under “—FATCA Withholding” and “—Information Reporting and Backup Withholding.”

Sale, Exchange or Other Disposition of Common Stock

A Non-U.S. Holder generally will not be subject to U.S. federal income tax on any gain recognized on the sale, exchange or other disposition of our common stock unless:

(i) such gain is effectively connected with the conduct of a trade or business in the United States by such Non-U.S. Holder, in which event such Non-U.S. Holder generally will be subject to U.S. federal income tax on such gain in substantially the same manner as a U.S. person (except as provided by an applicable tax treaty) and, if it is treated as a corporation for U.S. federal income tax purposes, may also be subject to a branch profits tax at a rate of 30% (or a lower rate if provided by an applicable tax treaty);

(ii) such Non-U.S. Holder is an individual who is present in the United States for 183 days or more during the taxable year of such sale, exchange or other disposition and certain other conditions are met, in which event such gain (net of certain U.S. source losses) generally will be subject to U.S. federal income tax at a rate of 30% (except as provided by an applicable tax treaty); or

(iii) we are or have been a “United States real property holding corporation” for U.S. federal income tax purposes at any time during the shorter of (x) the five-year period ending on the date of such sale, exchange or other disposition and (y) such Non-U.S. Holder’s holding period with respect to such common stock, and certain other conditions are met.

Generally, a corporation is a “United States real property holding corporation” if the fair market value of its United States real property interests equals or exceeds 50% of the sum of the fair market value of its worldwide real property interests and its other assets used or held for use in a trade or business (all as determined for U.S. federal income tax purposes). We believe that we presently are not, and we do not presently anticipate that we will become, a United States real property holding corporation. However, because this determination is made from time to time and is dependent upon a number of factors, some of which are beyond our control, including the value of our assets, there can be no assurance that we will not become a United States real property holding

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corporation. If we were a United States real property holding corporation during the period described in clause (iii) above, gain recognized by a Non-U.S. Holder generally would be treated as income effectively connected with the conduct of a trade or business in the United States by such Non-U.S. Holder, with the consequences described in clause (i) above (except that the branch profits tax would not apply), unless such Non-U.S. Holder owned (directly and constructively) five percent or less of our common stock throughout such period and our common stock is treated as “regularly traded on an established securities market” at any time during the calendar year of such sale, exchange or other disposition.

The foregoing discussion is subject to the discussion below under “—FATCA Withholding” and “—Information Reporting and Backup Withholding.”

FATCA Withholding

Under the Foreign Account Tax Compliance Act provisions of the Code and related U.S. Treasury guidance (“FATCA”), a withholding tax of 30% will be imposed in certain circumstances on payments of (i) dividends on our common stock and (ii) on or after January 1, 2019, gross proceeds from the sale or other disposition of our common stock. In the case of payments made to a “foreign financial institution” (such as a bank, a broker, an investment fund or, in certain cases, a holding company), as a beneficial owner or as an intermediary, this tax generally will be imposed, subject to certain exceptions, unless such institution (i) has agreed to (and does) comply with the requirements of an agreement with the United States (an “FFI Agreement”) or (ii) is required by (and does comply with) applicable foreign law enacted in connection with an intergovernmental agreement between the United States and a foreign jurisdiction (an “IGA”), in either case to, among other things, collect and provide to the U.S. tax authorities or other relevant tax authorities certain information regarding U.S. account holders of such institution and, in either case, such institution provides the withholding agent with a certification as to its FATCA status. In the case of payments made to a foreign entity that is not a financial institution (as a beneficial owner), the tax generally will be imposed, subject to certain exceptions, unless such entity provides the withholding agent with a certification as to its FATCA status and, in certain cases, identifies any “substantial” U.S. owner (generally, any specified U.S. person that directly or indirectly owns more than a specified percentage of such entity). If our common stock is held through a foreign financial institution that has agreed to comply with the requirements of an FFI Agreement or is subject to similar requirements under applicable foreign law enacted in connection with an IGA, such foreign financial institution (or, in certain cases, a person paying amounts to such foreign financial institution) generally will be required, subject to certain exceptions, to withhold tax on payments made to (i) a person (including an individual) that fails to provide any required information or documentation or (ii) a foreign financial institution that has not agreed to comply with the requirements of an FFI Agreement and is not subject to similar requirements under applicable foreign law enacted in connection with an IGA. Each Non-U.S. Holder should consult its own tax advisor regarding the application of FATCA to the ownership and disposition of our common stock.

Information Reporting and Backup Withholding

Amounts treated as payments of dividends on our common stock paid to a Non-U.S. Holder and the amount of any U.S. federal tax withheld from such payments generally must be reported annually to the IRS and to such Non-U.S. Holder by the applicable withholding agent.

The information reporting and backup withholding rules that apply to payments of dividends to certain U.S. persons generally will not apply to payments of dividends on our common stock to a Non-U.S. Holder if such Non-U.S. Holder certifies under penalties of perjury that it is not a U.S. person (generally by providing an IRS Form W-8BEN or W-8BEN-E to the applicable withholding agent) or otherwise establishes an exemption.

Proceeds from the sale, exchange or other disposition of our common stock by a Non-U.S. Holder effected outside the United States through a non-U.S. office of a non-U.S. broker generally will not be subject to the information reporting and backup withholding rules that apply to payments to certain U.S. persons, provided that

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the proceeds are paid to the Non-U.S. Holder outside the United States. However, proceeds from the sale, exchange or other disposition of our common stock by a Non-U.S. Holder effected through a non-U.S. office of a non-U.S. broker with certain specified U.S. connections or a U.S. broker generally will be subject to these information reporting rules (but generally not to these backup withholding rules), even if the proceeds are paid to such Non-U.S. Holder outside the United States, unless such Non-U.S. Holder certifies under penalties of perjury that it is not a U.S. person (generally by providing an IRS Form W-8BEN or W-8BEN-E to the applicable withholding agent) or otherwise establishes an exemption. Proceeds from the sale, exchange or other disposition of our common stock by a Non-U.S. Holder effected through a U.S. office of a broker generally will be subject to these information reporting and backup withholding rules unless such Non-U.S. Holder certifies under penalties of perjury that it is not a U.S. person (generally by providing an IRS Form W-8BEN or W-8BEN-E to the applicable withholding agent) or otherwise establishes an exemption.

Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules generally will be allowed as a refund or a credit against a Non-U.S. Holder’s U.S. federal income tax liability if the required information is furnished by such Non-U.S. Holder on a timely basis to the IRS.

U.S. Federal Estate Tax

Shares of our common stock owned or treated as owned by an individual Non-U.S. Holder at the time of such Non-U.S. Holder’s death will be included in such Non-U.S. Holder’s gross estate for U.S. federal estate tax purposes and may be subject to U.S. federal estate tax unless an applicable estate tax treaty provides otherwise.

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SELLING STOCKHOLDERS

The following table sets forth information as of April 27, 2018 with respect to the ownership of our common stock by the selling stockholders.

The amount and percentage of shares beneficially owned are reported on the basis of regulations of the SEC governing the determination of beneficial ownership of securities. Under SEC rules, a person is deemed to be a “beneficial owner” of a security if that person has or shares voting power or investment power, which includes the power to dispose of or to direct the disposition of such security. A person is also deemed to be a beneficial owner of any securities of which that person has a right to acquire beneficial ownership within 60 days. Securities that can be so acquired are deemed to be outstanding for purposes of computing such person’s ownership percentage, but not for purposes of computing any other person’s percentage. Under these rules, more than one person may be deemed to be a beneficial owner of the same securities, and a person may be deemed to be a beneficial owner of securities as to which such person has no economic interest.

Percentage computations are based on approximately 46,577,795 shares of our common stock outstanding as of April 27, 2018.

Except as otherwise indicated in these footnotes, the beneficial owner listed has, to our knowledge, sole voting and investment power with respect to the indicated shares of common stock.

	Shares Beneficially Owned Before the Offering and After the Offering
	Number of Shares Owned		Percent of Class Before the Offering (%)		Shares Offered Hereby		Percent of Class After the Offering (%)
CD&R Allied Holdings, L.P.(1)		6,959,976		14.9		6,959,976		—
Fidelity Investments Charitable Gift Fund(2)(3)		89,589		*		89,589		—
National Philanthropic Trust(2)(4)		185,273		*		185,273		—

* Less than 1%.

(1) CD&R Associates VIII, Ltd., or “CD&R Holdings GP”, as the general partner of the CD&R Investor, CD&R Associates VIII, L.P., as the sole shareholder of CD&R Holdings GP, and CD&R Investment Associates VIII, Ltd., as the general partner of CD&R Associates VIII, L.P., may each be deemed to beneficially own the shares of the Company’s common stock. Each of CD&R Holdings GP, CD&R Associates VIII, L.P. and CD&R Investment Associates VIII, Ltd. expressly disclaims beneficial ownership of the shares of the Company’s common stock in which the CD&R Investor has beneficial ownership. CD&R Investment Associates VIII, Ltd. is managed by a two-person board of directors. Donald J. Gogel and Kevin J. Conway, as the directors of CD&R Investment Associates VIII, Ltd., may be deemed to share beneficial ownership of the shares of the Company’s common stock in which the CD&R Investor has beneficial ownership. Such persons expressly disclaim such beneficial ownership. Investment and voting decisions with respect to the shares of the Company’s common stock held by the CD&R Investor are made by an investment committee of limited partners of CD&R Associates VIII, L.P., the “Investment Committee.” The CD&R investment professionals who have effective voting control of the Investment Committee are Michael G. Babiarz, Vindi Banga, James G. Berges, John C. Compton, Kevin J. Conway, Russell P. Fradin, Thomas C. Franco, Kenneth A. Giuriceo, Donald J. Gogel, Jillian Griffiths, Marco Herbst, Sarah Kim, John Krenicki, Jr., David A. Novak, Paul S. Pressler, Ravi Sachdev, Christian Rochat, Eric Rouzier, Richard J. Schnall, Stephen W. Shapiro, Nathan K. Sleeper, Derek L. Strum, David H. Wasserman and Jonathan L. Zrebiec. Messr. Sleeper is a director of Atkore. All members of the Investment Committee expressly disclaim beneficial ownership of the shares shown as beneficially owned by the CD&R Investor. The address for each of the CD&R Investor, CD&R Holdings GP, CD&R Associates VIII, L.P., and CD&R Investment Associates VIII, Ltd. is c/o Maples Corporate Services Limited, PO Box 309, Ugland House, South Church Street, George Town, Grand Cayman, KY1-1104, Cayman Islands.

(2) Represents shares received by such entity as a charitable contribution from certain CD&R investment professionals on May 16, 2018.

(3) The address of Fidelity Investments Charitable Gift Fund is 200 Seaport Boulevard, Z3B, Boston, Massachusetts 02210.

(4) The address of National Philanthropic Trust is 165 Township Line Road, Suite 1200, Jenkintown, PA 19046.

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UNDERWRITING

Credit Suisse Securities (USA) LLC is acting as the underwriter for this offering. Subject to the terms and conditions set forth in an underwriting agreement among us, the selling stockholders and the underwriter, the selling stockholder has agreed to sell to the underwriter, and the underwriter has agreed to purchase from the selling stockholders 7,234,838 shares of common stock.

Subject to the terms and conditions set forth in the underwriting agreement, the underwriter has agreed to purchase all of the shares sold under the underwriting agreement if any of these shares are purchased. If the underwriter defaults, the underwriting agreement provides that the underwriting agreement may be terminated.

We and the selling stockholders have agreed to indemnify the underwriter against certain liabilities, including liabilities under the Securities Act, or to contribute to payments the underwriter may be required to make in respect of those liabilities.

The underwriter is offering the shares, subject to prior sale, when, as and if accepted by the underwriter, subject to approval of legal matters by its counsel, including the validity of the shares, and other conditions contained in the underwriting agreement, such as the receipt by the underwriter of officer’s certificates and legal opinions. The underwriter reserves the right to withdraw, cancel or modify offers to the public and to reject orders in whole or in part.

Commissions and Discounts

The underwriter has agreed to purchase the shares of our common stock from the selling stockholders at a price of $19.91 per share, which will result in $144,045,624.58 of proceeds to the selling stockholders before expenses. The underwriter proposes to offer the shares of common stock from time to time for sale in one or more transactions on the NYSE, in the over-the-counter market, through negotiated transactions or otherwise at market prices prevailing at the time of sale, at prices related to prevailing market prices or at negotiated prices.

The fees and expenses of the offering are estimated at approximately $300,000 and are payable by us. We have also agreed to reimburse the underwriter for expenses relating to the clearance of this offering with the Financial Industry Regulatory Authority up to $25,000.

No Sales of Similar Securities

We, our directors and certain of our key executive officers have agreed not to sell or transfer any common stock or securities convertible into, exchangeable for, exercisable for, or repayable with common stock, for 30 days after the date of this prospectus supplement without first obtaining the written consent of the underwriter. Specifically, we, our directors and certain of our key executive officers have agreed, with certain limited exceptions, including, without limitation, with respect to sales or transfers pursuant to any trading plan in effect prior to the date of this prospectus supplement intended to comply with Rule 10b5-1 under the Securities Exchange Act of 1934, as amended, covering up to approximately 1.1 million shares of our common stock in the aggregate for the executive officers at varying prices not to directly or indirectly:

• offer, pledge, sell or contract to sell any common stock,

• sell any option or contract to purchase any common stock,

• purchase any option or contract to sell any common stock,

• grant any option, right or warrant for the sale of any common stock,

• lend or otherwise dispose of or transfer any common stock,

• request or demand that we file a registration statement related to the common stock, or

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• enter into any swap or other agreement that transfers, in whole or in part, the economic consequence of ownership of any common stock whether any such swap or transaction is to be settled by delivery of shares or other securities, in cash or otherwise.

This lock-up provision applies to common stock and to securities convertible into or exchangeable or exercisable for or repayable with common stock.

Listing

Our shares are listed on the NYSE under the symbol “ATKR.”

Price Stabilization and Short Positions

Until the distribution of the shares is completed, SEC rules may limit the underwriter and selling stockholders from bidding for and purchasing our common stock. However, the underwriter may engage in transactions that stabilize the price of the common stock, such as bids or purchases to peg, fix or maintain that price.

In connection with the offering, the underwriter may purchase and sell our common stock in the open market. These transactions may include short sales, purchases on the open market to cover positions created by short sales and stabilizing transactions. Short sales involve the sale by the underwriter of a greater number of shares than it is required to purchase in the offering. Stabilizing transactions consist of various bids for or purchases of shares of common stock made by the underwriter in the open market prior to the completion of the offering.

Similar to other purchase transactions, the underwriter’s purchases to cover the syndicate short sales may have the effect of raising or maintaining the market price of our common stock or preventing or retarding a decline in the market price of our common stock. As a result, the price of our common stock may be higher than the price that might otherwise exist in the open market. The underwriter may conduct these transactions on the NYSE, in the over-the-counter market or otherwise.

Neither we nor the underwriter make any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of our common stock. In addition, neither we nor the underwriter make any representations that the underwriter will engage in these transactions or that these transactions, once commenced, will not be discontinued without notice.

Passive Market Making

In connection with this offering, the underwriter and the selling stockholders may engage in passive market making transactions in the common stock on the NYSE in accordance with Rule 103 of Regulation M under the Exchange Act during a period before the commencement of offers or sales of common stock and extending through the completion of distribution. A passive market maker must display its bid at a price not in excess of the highest independent bid of that security. However, if all independent bids are lowered below the passive market maker’s bid, that bid must then be lowered when specified purchase limits are exceeded. Passive market making may cause the price of our common stock to be higher than the price that otherwise would exist in the open market in the absence of those transactions. The underwriter and dealers are not required to engage in passive market making and may end passive market making activities at any time.

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Electronic Distribution

In connection with the offering, the underwriter or securities dealers may distribute prospectuses by electronic means, such as e-mail.

Other Relationships

The underwriter and its affiliates have engaged in, and may in the future engage in, investment banking and other commercial dealings in the ordinary course of business with us or our affiliates. They have received customary fees for these transactions, including the customary fees provided to those acting as underwriter in our initial public offering, and may in the future receive customary fees and commissions for future transactions.

In addition, in the ordinary course of its business activities, the underwriter and its affiliates may make or hold a broad array of investments and actively trade debt and equity securities (or related derivative securities) and financial instruments (including bank loans) for their own account and for the accounts of their customers. Such investments and securities activities may involve securities and/or instruments of ours or our affiliates. The underwriter and its affiliates may also make investment recommendations and/or publish or express independent research views in respect of such securities or financial instruments and may hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

Notice to Prospective Investors in Canada

The shares of common stock may be sold only to purchasers purchasing, or deemed to be purchasing, as principal that are accredited investors, as defined in National Instrument 45-106 Prospectus Exemptions or subsection 73.3(1) of the Securities Act (Ontario), and are permitted clients, as defined in National Instrument 31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations. Any resale of the shares of common stock must be made in accordance with an exemption from, or in a transaction not subject to, the prospectus requirements of applicable securities laws.

Securities legislation in certain provinces or territories of Canada may provide a purchaser with remedies for rescission or damages if this prospectus supplement and accompanying prospectus (including any amendment thereto) contains a misrepresentation, provided that the remedies for rescission or damages are exercised by the purchaser within the time limit prescribed by the securities legislation of the purchaser’s province or territory. The purchaser should refer to any applicable provisions of the securities legislation of the purchaser’s province or territory for particulars of these rights or consult with a legal advisor.

Pursuant to section 3A.3 (or, in the case of securities issued or guaranteed by the government of a non-Canadian jurisdiction, section 3A.4) of National Instrument 33-105 Underwriting Conflicts (NI 33-105), the underwriter is not required to comply with the disclosure requirements of NI 33-105 regarding underwriter conflicts of interest in connection with this offering.

Notice to Prospective Investors in the European Economic Area

In relation to each Member State of the European Economic Area that has implemented the Prospectus Directive (each, a “Relevant Member State”), with effect from and including the date on which the Prospectus Directive is implemented in that Relevant Member State (the “Relevant Implementation Date”), no offer of shares may be made to the public in that Relevant Member State other than:

A. to any legal entity which is a qualified investor as defined in the Prospectus Directive;

B. to fewer than 100 or, if the Relevant Member State has implemented the relevant provision of the 2010 PD Amending Directive, 150, natural or legal persons (other than qualified investors as defined in the Prospectus Directive), as permitted under the Prospectus Directive, subject to obtaining the prior consent of the underwriter; or

C. in any other circumstances falling within Article 3(2) of the Prospectus Directive;

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provided that no such offer of shares shall require the Company or the underwriter to publish a prospectus pursuant to Article 3 of the Prospectus Directive or supplement a prospectus pursuant to Article 16 of the Prospectus Directive.

Each person in a Relevant Member State who initially acquires any shares or to whom any offer is made will be deemed to have represented, acknowledged and agreed that (A) it is a “qualified investor” within the meaning of the law in that Relevant Member State implementing Article 2(1)(e) of the Prospectus Directive, and (B) in the case of any shares acquired by it as a financial intermediary, as that term is used in Article 3(2) of the Prospectus Directive, the shares acquired by it in the offering have not been acquired on behalf of, nor have they been acquired with a view to their offer or resale to, persons in any Relevant Member State other than “qualified investors” as defined in the Prospectus Directive, or in circumstances in which the prior consent of the underwriter has been given to the offer or resale. In the case of any shares being offered to a financial intermediary as that term is used in Article 3(2) of the Prospectus Directive, each such financial intermediary will be deemed to have represented, acknowledged and agreed that the shares acquired by it in the offer have not been acquired on a non-discretionary basis on behalf of, nor have they been acquired with a view to their offer or resale to, persons in circumstances which may give rise to an offer of any shares to the public other than their offer or resale in a Relevant Member State to qualified investors as so defined or in circumstances in which the prior consent of the underwriter has been obtained to each such proposed offer or resale.

The Company, the underwriter and their affiliates will rely upon the truth and accuracy of the foregoing representation, acknowledgement and agreement.

This prospectus supplement and accompanying prospectus have been prepared on the basis that any offer of shares in any Relevant Member State will be made pursuant to an exemption under the Prospectus Directive from the requirement to publish a prospectus for offers of shares. Accordingly any person making or intending to make an offer in that Relevant Member State of shares that are the subject of the offering contemplated in this prospectus supplement may only do so in circumstances in which no obligation arises for the Company or the underwriter to publish a prospectus pursuant to Article 3 of the Prospectus Directive in relation to such offer. Neither the Company nor the underwriter has authorized, nor do they authorize, the making of any offer of shares in circumstances in which an obligation arises for the Company or the underwriter to publish a prospectus for such offer.

For the purpose of the above provisions, the expression “an offer to the public” in relation to any shares in any Relevant Member State means the communication in any form and by any means of sufficient information on the terms of the offer and the shares to be offered so as to enable an investor to decide to purchase or subscribe the shares, as the same may be varied in the Relevant Member State by any measure implementing the Prospectus Directive in the Relevant Member State and the expression “Prospectus Directive” means Directive 2003/71/EC (including the 2010 PD Amending Directive, to the extent implemented in the Relevant Member States) and includes any relevant implementing measure in the Relevant Member State and the expression “2010 PD Amending Directive” means Directive 2010/73/EU.

Notice to Prospective Investors in the United Kingdom

In addition, in the United Kingdom, this document is being distributed only to, and is directed only at, and any offer subsequently made may only be directed at persons who are “qualified investors” (as defined in the Prospectus Directive) (i) who have professional experience in matters relating to investments falling within Article 19 (5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005, as amended (the “Order”) and/or (ii) who are high net worth companies (or persons to whom it may otherwise be lawfully communicated) falling within Article 49(2)(a) to (d) of the Order (all such persons together being referred to as “relevant persons”). This document must not be acted on or relied on in the United Kingdom by persons who are not relevant persons. In the United Kingdom, any investment or investment activity to which this document relates is only available to, and will be engaged in with, relevant persons.

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Notice to Prospective Investors in Switzerland

The shares may not be publicly offered in Switzerland and will not be listed on the SIX Swiss Exchange (“SIX”) or on any other stock exchange or regulated trading facility in Switzerland. This document has been prepared without regard to the disclosure standards for issuance prospectuses under art. 652a or art. 1156 of the Swiss Code of Obligations or the disclosure standards for listing prospectuses under art. 27 ff. of the SIX Listing Rules or the listing rules of any other stock exchange or regulated trading facility in Switzerland. Neither this document nor any other offering or marketing material relating to the shares or the offering may be publicly distributed or otherwise made publicly available in Switzerland.

Neither this document nor any other offering or marketing material relating to the offering, the Company, or the shares have been or will be filed with or approved by any Swiss regulatory authority. In particular, this document will not be filed with, and the offer of shares will not be supervised by, the Swiss Financial Market Supervisory Authority FINMA, and the offer of shares has not been and will not be authorized under the Swiss Federal Act on Collective Investment Schemes (“CISA”). The investor protection afforded to acquirers of interests in collective investment schemes under the CISA does not extend to acquirers of shares.

Notice to Prospective Investors in the Dubai International Financial Centre

This prospectus supplement and accompanying prospectus relate to an Exempt Offer in accordance with the Offered Securities Rules of the Dubai Financial Services Authority (“DFSA”). This prospectus supplement and accompanying prospectus are intended for distribution only to persons of a type specified in the Offered Securities Rules of the DFSA. It must not be delivered to, or relied on by, any other person. The DFSA has no responsibility for reviewing or verifying any documents in connection with Exempt Offers. The DFSA has not approved this prospectus supplement and accompanying prospectus nor taken steps to verify the information set forth herein and therein and has no responsibility for the prospectus supplement and accompanying prospectus. The shares to which this prospectus supplement and accompanying prospectus relate may be illiquid and/or subject to restrictions on their resale. Prospective purchasers of the shares offered should conduct their own due diligence on the shares. If you do not understand the contents of this prospectus supplement and accompanying prospectus you should consult an authorized financial advisor.

Notice to Prospective Investors in Australia

No placement document, prospectus, product disclosure statement or other disclosure document has been lodged with the Australian Securities and Investments Commission, in relation to the offering. This prospectus supplement and accompanying prospectus do not constitute a prospectus, product disclosure statement or other disclosure document under the Corporations Act 2001 (the “Corporations Act”), and do not purport to include the information required for a prospectus, product disclosure statement or other disclosure document under the Corporations Act.

Any offer in Australia of the shares may only be made to persons (the “Exempt Investors”) who are “sophisticated investors” (within the meaning of section 708(8) of the Corporations Act), “professional investors” (within the meaning of section 708(11) of the Corporations Act) or otherwise pursuant to one or more exemptions contained in section 708 of the Corporations Act so that it is lawful to offer the shares without disclosure to investors under Chapter 6D of the Corporations Act.

The shares applied for by Exempt Investors in Australia must not be offered for sale in Australia in the period of 12 months after the date of allotment under the offering, except in circumstances where disclosure to investors under Chapter 6D of the Corporations Act would not be required pursuant to an exemption under section 708 of the Corporations Act or otherwise or where the offer is pursuant to a disclosure document that complies with Chapter 6D of the Corporations Act. Any person acquiring shares must observe such Australian on-sale restrictions.

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This prospectus supplement and accompanying prospectus contain general information only and does not take account of the investment objectives, financial situation or particular needs of any particular person. It does not contain any securities recommendations or financial product advice. Before making an investment decision, investors need to consider whether the information in this prospectus supplement and accompanying prospectus is appropriate to their needs, objectives and circumstances, and, if necessary, seek expert advice on those matters.

Notice to Prospective Investors in Hong Kong

The shares have not been offered or sold and will not be offered or sold in Hong Kong, by means of any document, other than (a) to “professional investors” as defined in the Securities and Futures Ordinance (Cap. 571) of Hong Kong and any rules made under that Ordinance; or (b) in other circumstances which do not result in the document being a “prospectus” as defined in the Companies Ordinance (Cap. 32) of Hong Kong or which do not constitute an offer to the public within the meaning of that Ordinance. No advertisement, invitation or document relating to the shares has been or may be issued or has been or may be in the possession of any person for the purpose of issue, whether in Hong Kong or elsewhere, which is directed at, or the contents of which are likely to be accessed or read by, the public in Hong Kong (except if permitted to do so under the laws of Hong Kong) other than with respect to shares that are or are intended to be disposed of only to persons outside Hong Kong or only to “professional investors” within the meaning of the Securities and Futures Ordinance and any rules made thereunder.

Notice to Prospective Investors in Japan

The shares have not been and will not be registered under the Financial Instruments and Exchange Law of Japan (Law No. 25 of 1948, as amended) and, accordingly, will not be offered or sold, directly or indirectly, in Japan, or for the benefit of any Japanese Person or to others for re-offering or resale, directly or indirectly, in Japan or to any Japanese Person, except in compliance with all applicable laws, regulations and ministerial guidelines promulgated by relevant Japanese governmental or regulatory authorities in effect at the relevant time. For the purposes of this paragraph, “Japanese Person” shall mean any person resident in Japan, including any corporation or other entity organized under the laws of Japan.

Notice to Prospective Investors in Singapore

This prospectus supplement and accompanying prospectus have not been registered as a prospectus with the Monetary Authority of Singapore. Accordingly, this prospectus supplement and accompanying prospectus and any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of the shares may not be circulated or distributed, nor may the shares be offered or sold, or be made the subject of an invitation for subscription or purchase, whether directly or indirectly, to persons in Singapore other than (i) to an institutional investor under Section 274 of the Securities and Futures Act, Chapter 289 of Singapore (the “SFA”), (ii) to a relevant person pursuant to Section 275(1), or any person pursuant to Section 275(1A), and in accordance with the conditions specified in Section 275 of the SFA or (iii) otherwise pursuant to, and in accordance with the conditions of, any other applicable provision of the SFA.

Where the shares are subscribed or purchased under Section 275 of the SFA by a relevant person that is: (i) a corporation (that is not an accredited investor (as defined in Section 4A of the SFA)) the sole business of which is to hold investments and the entire share capital of which is owned by one or more individuals, each of whom is an accredited investor; or (ii) a trust (where the trustee is not an accredited investor) whose sole purpose is to hold investments and each beneficiary of the trust is an individual who is an accredited investor, securities (as defined in Section 239(1) of the SFA) of that corporation or the beneficiaries’ rights and interest (howsoever described) in that trust shall not be transferred within six months after that corporation or that trust has acquired the shares pursuant to an offer made under Section 275 of the SFA except: (a) to an institutional investor or to a relevant person defined in Section 275(2) of the SFA, or to any person arising from an offer referred to in Section 275(1A) or Section 276(4)(i)(B) of the SFA; (b) where no consideration is or will be given for the transfer; (c) where the transfer is by operation of law; (d) as specified in Section 276(7) of the SFA; or (e) as specified in Regulation 32 of the Securities and Futures (Offers of Investments) (Shares and Debentures) Regulations 2005 of Singapore.

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LEGAL MATTERS

The validity of the shares of common stock offered in this offering will be passed upon for us by Debevoise & Plimpton LLP, New York, New York. Certain legal matters relating to this offering will be passed upon for the underwriter by Latham & Watkins LLP, New York, New York.

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EXPERTS

The consolidated financial statements, and the related financial statement schedules, incorporated in this prospectus supplement by reference from the Company’s Annual Report on Form 10-K, and the effectiveness of Atkore International Group Inc.’s internal control over financial reporting have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their reports which are incorporated herein by reference. Such consolidated financial statements and financial statement schedules have been so incorporated in reliance upon the report of such firm given upon their authority as experts in accounting and auditing.

WHERE YOU CAN FIND MORE INFORMATION

We have filed a registration statement on Form S-3 with the SEC with respect to the shares of our common stock being sold in this offering. This prospectus supplement and accompanying prospectus do not contain all of the information set forth in the registration statement, the post-effective amendments and the exhibits thereto because some parts have been omitted in accordance with the rules and regulations of the SEC. You will find additional information about us and the common stock being sold in this offering in the registration statement, the post-effective amendments and the exhibits thereto. For further information with respect to Atkore and the common stock offered hereby, we refer you to the registration statement, the post-effective amendments and the exhibits filed therewith. Statements contained in this prospectus supplement and the documents incorporated by reference into this prospectus supplement as to the contents of any contract or other document referred to are not necessarily complete and in each instance, if such contract or document is filed as an exhibit, reference is made to the copy of such contract or other document filed as an exhibit to the registration statement, each statement being qualified in all respects by such reference. A copy of the registration statement and the post-effective amendments, including the exhibits thereto, may be read and copied at the SEC’s Public Reference Room at 100 F Street, N.E., Room 1580, Washington, D.C. 20549. Information on the operation of the Public Reference Room may be obtained by calling the SEC at 1-800-SEC-0330. In addition, the SEC maintains an internet site at http://www.sec.gov, from which interested persons can electronically access the registration statement and the post-effective amendments, including the exhibits and any schedules thereto. Copies of the registration statement and the post-effective amendments, including the exhibits and schedules thereto, are also available at your request, without charge, from Atkore International Group Inc., 16100 South Lathrop Avenue, Harvey, Illinois 60426, Attention: Legal Department.

We are subject to the informational requirements of the Exchange Act and, accordingly, file annual reports containing financial statements audited by an independent registered public accounting firm, quarterly reports containing unaudited financial statements, current reports, proxy statements and other information with the SEC. You may inspect and copy these reports, proxy statements and other information at the public reference facilities maintained by the SEC at the address noted above. You may also obtain copies of this material from the Public Reference Room of the SEC as described above, or inspect them without charge at the SEC’s website. You may access, free of charge, our reports filed with the SEC (for example, our Annual Reports on Form 10-K, our Quarterly Reports on Form 10-Q and our Current Reports on Form 8-K and any amendments to those forms) through our website (http://investors.atkore.com). Reports filed with or furnished to the SEC will be available as soon as reasonably practicable after they are filed with or furnished to the SEC. None of the information contained on, or that may be accessed through, our websites or any other website identified herein is part of, or incorporated into, this prospectus supplement. All website addresses in this prospectus supplement are intended to be inactive textual references only.

INCORPORATION BY REFERENCE

The SEC allows us to “incorporate by reference” into this prospectus supplement information we file with the SEC in other documents. This means that we can disclose important information to you by referring to

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another document we have filed with the SEC. The information relating to us contained in this prospectus supplement and the accompanying prospectus should be read together with the information in the documents incorporated by reference.

We incorporate by reference, as of their respective dates of filing, the documents listed below (excluding any portions of such documents that have been “furnished” but not “filed” for purposes of the Exchange Act):

• our Annual Report on Form 10-K for the year ended September 30, 2017, filed with the SEC on November 29, 2017;

• our Quarterly Reports on Form 10-Q for the quarters ended December 31, 2017 (filed with the SEC on February 6, 2018) and March 30, 2018 (filed with the SEC on May 8, 2018);

• our Current Reports on Form 8-K, filed with the SEC on May 8, 2018, May 4, 2018, February 21, 2018, February 5, 2018, February 2, 2018, February 1, 2018, January 24, 2018 and January 22, 2018 (excluding any information furnished under Item 2.02 or Item 7.01);

• our Definitive Proxy Statement on Schedule 14A filed with the SEC on December 15, 2017 (only those parts incorporated in our Annual Report on Form 10-K for the year ended September 30, 2017);

• the description of capital stock contained in the Registration Statement on Form 8-A, as filed with the SEC on June 1, 2016, as supplemented by the “Description of Capital Stock” found on page 6 of the prospectus and including any amendments or reports filed for the purpose of updating such description; and

• all documents filed by us with the SEC pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act, as amended (other than Current Reports on Form 8-K furnished under Items 2.02 and 7.01 (including any financial statements or exhibits relating thereto furnished pursuant to Item 9.01) of Form 8-K and not specifically incorporated by reference), after the date on which Post-Effective Amendment No. 2 to the registration statement to which this prospectus supplement relates was initially filed with the SEC and prior to the termination of the offering to which this prospectus supplement relates.

Any statement incorporated by reference in this prospectus supplement from an earlier dated document that is inconsistent with a statement contained in this prospectus supplement or in any other document filed after the date of the earlier dated document, but prior to the termination of the offering to which this prospectus supplement relates, which also is incorporated by reference into this prospectus supplement, shall be deemed to be modified or superseded for purposes of this prospectus supplement by such statement contained in this prospectus supplement or in any other document filed after the date of the earlier dated document, but prior to the termination of the offering to which this prospectus supplement relates, which also is incorporated by reference into this prospectus supplement.

Any person, including any beneficial owner, to whom this prospectus supplement is delivered may request copies of this prospectus supplement and any of the documents incorporated by reference into this prospectus supplement, without charge, by written or oral request directed to Atkore International Group Inc., 16100 South Lathrop Avenue, Harvey, Illinois 60426, Attention: Investor Relations Dept., Telephone: (708) 339-1610 or the investor relations page of our website at http://investors.atkore.com, or from the SEC through the SEC’s Internet website at the address provided under “Where You Can Find More Information”. All other information contained on our website is not a part of this prospectus supplement. Documents incorporated by reference into this prospectus supplement are available without charge, excluding any exhibits to those documents unless the exhibit is specifically incorporated by reference into those documents.

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Atkore International Group Inc.

Common Stock

The selling stockholder named in this prospectus may offer and sell shares of common stock of Atkore International Group Inc. from time to time up to 30,460,377 shares of common stock, at prices and on terms that will be determined at the time of the offering.

Each time shares of common stock are offered pursuant to this prospectus, we will provide a prospectus supplement and attach it to this prospectus. The prospectus supplement will contain more specific information about the offering. The prospectus supplement may also add, update or change information contained in this prospectus. This prospectus may not be used to offer or sell securities without a prospectus supplement describing the method and terms of the offering.

You should carefully read this prospectus and any accompanying prospectus supplement, together with the documents we incorporate by reference, before you invest in our common stock.

Investing in our common stock involves risks. See the section entitled “Risk Factors” on page 6 of this prospectus and any risk factors described in any applicable prospectus supplement and in the documents we incorporate by reference.

The common stock of Atkore International Group Inc. is listed on the New York Stock Exchange under the symbol “ATKR”. The last reported sale price of our common stock on November 29, 2017 was $22.03 per share.

Neither the U.S. Securities and Exchange Commission (the “SEC”) nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus is December 15, 2017.

TABLE OF CONTENTS

About This Prospectus	1
Special Note Regarding Forward-Looking Statements and Information	2
Our Company	5
Risk Factors	6
Use of Proceeds	6
Description of Capital Stock	6
Selling Stockholder	12
Plan of Distribution	13
U.S. Federal Income Tax Considerations for Non-U.S. Holders	16
Legal Matters	20
Experts	20
Where You Can Find More Information	20
Incorporation of Certain Information by Reference	21

ABOUT THIS PROSPECTUS

Unless the context otherwise requires, references in this prospectus to the “Company”, “ATKR”, “we”, “us” and “our” mean Atkore International Group Inc.

This prospectus is part of a registration statement on Form S-3 that we filed with the SEC utilizing a “shelf” registration process. Under this shelf registration process, the selling stockholder over time may offer and sell up to 30,460,377 shares of our common stock in one or more offerings or resales. This prospectus provides you with a general description of the shares of common stock the selling stockholder may offer. Each time the selling stockholder sells shares of common stock, we will, to the extent required, provide a prospectus supplement that will contain specific information about the terms of that offering. The prospectus supplement and any free writing prospectus may also add to, update, supplement or clarify information contained or incorporated by reference in this prospectus. Any statement that we make in this prospectus will be modified or superseded by any inconsistent statement made by us in a prospectus supplement.

The rules of the SEC allow us to incorporate information by reference into this prospectus. This information incorporated by reference is considered to be part of this prospectus, and information that we file later with the SEC will automatically update and supersede this information. See “Incorporation of Certain Information by Reference.” You should read both this prospectus and any prospectus supplement together with additional information described under the heading “Where You Can Find More Information.”

Neither we, the selling stockholder nor any underwriters have authorized anyone to provide you with different information or to make any representations other than those contained or incorporated by reference into this prospectus or in any free writing prospectuses we have prepared. If anyone provides you with different or inconsistent information, you should not rely on it. This prospectus does not constitute an offer to sell, or a solicitation of an offer to purchase, the securities offered by this prospectus in any jurisdiction in which it is unlawful to make such offer or solicitation.

You should not assume that the information incorporated by reference or provided in this prospectus or any applicable prospectus supplement or any free writing prospectus prepared by us is accurate as of any date other than the date on the front cover of those documents. Our business, financial condition, results of operations and prospectus may have changed since that date.

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS AND INFORMATION

This prospectus, the accompanying prospectus supplements and the documents incorporated by reference herein and therein contain forward-looking statements and cautionary statements within the meaning of the Private Securities Litigation Reform Act of 1995 that are based on management’s beliefs and assumptions and information currently available to management. Some of the forward-looking statements can be identified by the use of forward-looking terms such as “believes,” “expects,” “may,” “will,” “shall,” “should,” “would,” “could,” “seeks,” “aims,” “projects,” “is optimistic,” “intends,” “plans,” “estimates,” “anticipates” or other comparable terms. Forward-looking statements include, without limitation, all matters that are not historical facts. They appear in a number of places throughout this prospectus, the applicable prospectus supplement or in the documents incorporated by reference herein or therein and include, without limitation, statements regarding our intentions, beliefs, assumptions or current expectations concerning, among other things, financial position; results of operations; cash flows; prospects; growth strategies or expectations; timing and amount of share repurchases; customer retention; the outcome (by judgment or settlement) and costs of legal, administrative or regulatory proceedings, investigations or inspections, including, without limitation, collective, representative or class action litigation; and the impact of prevailing economic conditions.

Forward-looking statements are subject to known and unknown risks and uncertainties, many of which may be beyond our control. We caution you that forward-looking statements are not guarantees of future performance or outcomes and that actual performance and outcomes, including, without limitation, our actual results of operations, financial condition and liquidity, and the development of the market in which we operate, may differ materially from those made in or suggested by the forward-looking statements contained in this prospectus, the applicable prospectus supplement and the documents incorporated by reference herein or therein. In addition, even if our results of operations, financial condition and cash flows, and the development of the market in which we operate, are consistent with the forward-looking statements contained in this prospectus, the applicable prospectus supplement and the documents incorporated by reference herein or therein, those results or developments may not be indicative of results or developments in subsequent periods. A number of important factors, including, without limitation, the risks and uncertainties discussed under the caption “Risk Factors” in this prospectus, the applicable prospectus supplement and those described from time to time in our other filings with the SEC, could cause actual results and outcomes to differ materially from those reflected in the forward-looking statements. Additional factors that could cause actual results and outcomes to differ from those reflected in forward-looking statements include, without limitation:

• declines in, and uncertainty regarding, the general business and economic conditions in the U.S. and international markets in which we operate;

• weakness or another downturn in the U.S. non-residential construction industry;

• changes in prices of raw materials;

• pricing pressure, reduced profitability, or loss of market share due to intense competition;

• availability and cost of third-party freight carriers and energy;

• high levels of imports of products similar to those manufactured by us;

• changes in federal, state, local and international governmental regulations and trade policies;

• adverse weather conditions;

• failure to generate sufficient cash flow from operations or to raise sufficient funds in the capital markets to satisfy existing obligations and support the development of our business;

• increased costs relating to future capital and operating expenditures to maintain compliance with environmental, health and safety laws;

• reduced spending by, deterioration in the financial condition of, or other adverse developments with respect to, one or more of our top customers;

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• increases in our working capital needs, which are substantial and fluctuate based on economic activity and the market prices for our main raw materials, including as a result of failure to collect, or delays in the collection of, cash from the sale of manufactured products;

• work stoppage or other interruptions of production at our facilities as a result of disputes under existing collective bargaining agreements with labor unions or, in connection with negotiations of new collective bargaining agreements, as a result of supplier financial distress, or for other reasons;

• challenges attracting and retaining key personnel or high-quality employees;

• changes in our financial obligations relating to pension plans that we maintain in the United States;

• reduced production or distribution capacity due to interruptions in the operations of our facilities or those of our key suppliers;

• loss of a substantial number of our third-party agents or distributors or a dramatic deviation from the amount of sales they generate;

• security threats, attacks, or other disruptions to our information systems, or failure to comply with complex network security, data privacy and other legal obligations or the failure to protect sensitive information;

• possible impairment of goodwill or other long-lived assets as a result of future triggering events, such as declines in our cash flow projections or customer demand;

• safety and labor risks associated with the manufacture and in the testing of our products;

• product liability, construction defect and warranty claims and litigation relating to our various products, as well as government inquiries and investigations, and consumer, employment, tort and other legal proceedings;

• our ability to protect our intellectual property and other material proprietary rights;

• risks inherent in doing business internationally;

• our inability to introduce new products effectively or implement our innovation strategies;

• the inability of our customers to pay off the credit lines extended to them by us in a timely manner and the negative impact on customer relations resulting from our collections efforts with respect to non- paying or slow-paying customers;

• our inability to continue importing raw materials, component parts and/or finished goods;

• changes as a result of comprehensive tax reform;

• the incurrence of liabilities and the issuance of additional debt or equity in connection with acquisitions, joint ventures or divestitures and the failure of indemnification provisions in our acquisition agreements to fully protect us from unexpected liabilities;

• failure to manage acquisitions successfully, including identifying, evaluating, and valuing acquisition targets and integrating acquired companies, businesses or assets;

• the incurrence of liabilities in connection with violations of the FCPA and similar foreign anti- corruption laws;

• the incurrence of additional expenses, increase in complexity of our supply chain and potential damage to our reputation with customers resulting from regulations related to “conflict minerals”;

• disruptions or impediments to the receipt of sufficient raw materials resulting from various anti- terrorism security measures;

• restrictions contained in our debt agreements;

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• failure to generate cash sufficient to pay the principal of, interest on, or other amounts due on our debt;

• the significant influence the CD&R Investor will have over our corporate decisions; and

• other risks and factors included under “Risk Factors” and elsewhere in this prospectus.

You should read this prospectus, and the applicable prospectus supplement, including the uncertainties and factors discussed under “Risk Factors” and the documents incorporated by reference herein and therein completely and with the understanding that actual future results may be materially different from expectations. All forward-looking statements attributable to us or persons acting on our behalf that are made in this prospectus and the applicable prospectus supplement and the documents incorporated by reference herein or therein are qualified in their entirety by these cautionary statements. These forward-looking statements are made only as of the date presented, and we do not undertake any obligation, other than as may be required by law, to update or revise any forward-looking or cautionary statements to reflect changes in assumptions, the occurrence of events, unanticipated or otherwise, and changes in future operating results over time or otherwise.

Comparisons of results between current and prior periods are not intended to express any future trends, or indications of future performance, unless expressed as such, and should only be viewed as historical data.

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OUR COMPANY

We are a leading manufacturer of Electrical Raceway products primarily for the non-residential construction and renovation markets and Mechanical Products & Solutions for the construction and industrial markets. Electrical Raceway products form the critical infrastructure that enables the deployment, isolation and protection of a structure’s electrical circuitry from the original power source to the final outlet. Mechanical Products & Solutions frame, support and secure component parts in a broad range of structures, equipment and systems in electrical, industrial and construction applications. The quality of our products, the strength of our brands and our scale and national presence provide what we believe to be a unique set of competitive advantages that positions us for profitable growth.

We manufacture a broad range of end-to-end integrated products and solutions that are critical to our customers’ businesses. Our broad product offering enables us to bundle and co-load a wide range of products, which simplifies the ordering and delivery processes and streamlines logistics, reducing costs for us and our customers. We primarily serve electrical contractors and original equipment manufacturers, both directly and through our established core customer base of electrical and industrial distributors. Our operational footprint, together with our national distribution network, provides important proximity to our customers and enables efficient and reliable delivery of our products. Our scale creates meaningful purchasing power with key suppliers and enables us to leverage common manufacturing technology and processes across our business.

Our principal executive offices are located at 16100 South Lathrop Avenue, Harvey, Illinois 60426, and our telephone number at that address is (708) 339-1610.

5

RISK FACTORS

Investing in our common stock involves a high degree of risk. Before you make your investment decision, you should carefully consider all the information contained or incorporated by reference in this prospectus and any accompanying prospectus supplement. In particular, we urge you to consider carefully the risks and uncertainties discussed in “Part I—Item 1A—Risk Factors” of our Annual Report on Form 10-K for the year ended September 30, 2017, as such risk factors may be updated by our annual, quarterly and current reports that we may file with the SEC after the date on which Post-Effective Amendment No. 2 to the registration statement that includes this prospectus was initially filed with the SEC and that are incorporated by reference in this prospectus and any accompanying prospectus supplement.

USE OF PROCEEDS

We will not receive any proceeds from any sale of shares of our common stock by the selling stockholder. The selling stockholder will bear any underwriting commissions and discounts attributable to its sale of our common stock, and we will bear the remaining expenses.

DESCRIPTION OF CAPITAL STOCK

The following descriptions of our capital stock, Second Amended and Restated Certificate of Incorporation (“amended and restated certificate of incorporation”) and Second Amended and Restated By-laws (“amended and restated by-laws”) are intended as summaries only and are qualified in their entirety by reference to our amended and restated certificate of incorporation and amended and restated by-laws.

General

Our authorized capital stock consists of 1,000,000,000 shares of common stock, par value $0.01 per share and 100,000,000 shares of preferred stock, par value $1.00 per share. As of November 17, 2017, there were 63,090,619 shares of our common stock issued and outstanding not including 5,013,092 shares of our common stock issuable upon exercise of outstanding stock options, 433,107 shares of our common stock subject to outstanding restricted stock units and 181,236 shares of our common stock subject to outstanding performance shares.

Common Stock

Holders of common stock are entitled:

• to cast one vote for each share held of record on all matters submitted to a vote of the stockholders;

• to receive, on a pro rata basis, dividends and distributions, if any, that our board of directors may declare out of legally available funds, subject to preferences that may be applicable to preferred stock, if any, then outstanding; and

• upon our liquidation, dissolution or winding up, to share equally and ratably in any assets remaining after the payment of all debt and other liabilities, subject to the prior rights, if any, of holders of any outstanding shares of preferred stock.

Our ability to pay dividends on our common stock is subject to our subsidiaries’ ability to pay dividends to us, which is in turn subject to the restrictions set forth in the agreements that govern our indebtedness.

The holders of our common stock do not have any preemptive, cumulative voting, subscription, conversion, redemption or sinking fund rights. The common stock is not subject to future calls or assessments by us. The rights and privileges of holders of our common stock are subject to any series of preferred stock that we may issue in the future, as described below.

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As of November 17, 2017, we had 63,090,619 shares of common stock outstanding and there were 2 holders of record of our common stock.

Preferred Stock

Under our amended and restated certificate of incorporation, our board of directors has the authority, without further action by our stockholders, to issue up to 100,000,000 shares of preferred stock in one or more series and to fix the powers, designations, preferences and the relative participating, optional or other special rights and qualifications, limitations and restrictions of each series, including dividend rights, dividend rates, conversion rights, voting rights, terms of redemption, liquidation preferences and the number of shares constituting any series. No shares of our authorized preferred stock are currently outstanding. Because the board of directors has the power to establish the preferences and rights of the shares of any additional series of preferred stock, it may afford holders of any preferred stock preferences, powers and rights, including voting and dividend rights, senior to the rights of holders of our common stock, which could adversely affect the holders of the common stock and could delay, discourage or prevent a takeover of us even if a change of control of our company would be beneficial to the interests of our stockholders.

Annual Stockholders Meeting

Our amended and restated by-laws provide that annual stockholder meetings will be held at a date, time and place, if any, as exclusively selected by our board of directors. To the extent permitted under applicable law, we may conduct meetings by remote communications, including by webcast.

Voting

The affirmative vote of a plurality of the shares of our common stock present, in person or by proxy, at the meeting and entitled to vote on the election of directors will decide the election of any directors, and the affirmative vote of a majority of the shares of our common stock present, in person or by proxy, at the meeting and entitled to vote at any annual or special meeting of stockholders will decide all other matters voted on by stockholders, unless the question is one upon which, by express provision of law, under our amended and restated certificate of incorporation, or under our amended and restated by-laws, a different vote is required, in which case such provision will control.

Anti-Takeover Effects of Our Amended and Restated Certificate of Incorporation and Amended and Restated By-Laws

The provisions of our amended and restated certificate of incorporation and amended and restated by-laws summarized below may have an anti-takeover effect and may delay, defer or prevent a tender offer or takeover attempt that you might consider in your best interest, including an attempt that might result in your receipt of a premium over the market price for your shares. These provisions are also designed, in part, to encourage persons seeking to acquire control of us to first negotiate with our board of directors, which could result in an improvement of their terms.

Authorized but Unissued Shares of Common Stock. Under the Delaware General Corporate Law (the “DGCL”), our board of directors has the authority to issue the remaining shares of our authorized and unissued common stock without additional stockholder approval, subject to compliance with applicable New York Stock Exchange (“NYSE”) requirements. While the additional shares are not designed to deter or prevent a change of control, under some circumstances we could use the additional shares to create voting impediments or to frustrate persons seeking to effect a takeover or otherwise gain control by, for example, issuing those shares in private placements to purchasers who might side with our board of directors in opposing a hostile takeover bid.

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Authorized but Unissued Shares of Preferred Stock. Under our amended and restated certificate of incorporation, our board of directors has the authority, without further action by our stockholders, to issue up to 100,000,000 shares of preferred stock in one or more series and to fix the powers, designations, preferences and the relative participating, optional or other special rights and qualifications, limitations and restrictions of each series, including dividend rights, dividend rates, conversion rights, voting rights, terms of redemption, liquidation preferences and the number of shares constituting any series. The existence of authorized but unissued preferred stock could reduce our attractiveness as a target for an unsolicited takeover bid since we could, for example, issue shares of preferred stock to parties who might oppose such a takeover bid or shares that contain terms the potential acquiror may find unattractive. This may have the effect of delaying or preventing a change of control, may discourage bids for the common stock at a premium over the market price of the common stock, and may adversely affect the market price of, and the voting and other rights of the holders of, our common stock.

Classified Board of Directors. In accordance with the terms of our amended and restated certificate of incorporation, our board of directors is divided into three classes, Class I, Class II and Class III, with members of each class serving staggered three-year terms. Under our amended and restated certificate of incorporation, our board of directors consists of such number of directors as may be determined from time to time by resolution of the board of directors, but in no event may the number of directors be less than one. Any additional directorships resulting from an increase in the number of directors will be distributed among the three classes so that, as nearly as possible, each class will consist of one-third of the directors. Our amended and restated certificate of incorporation also provides that any vacancy on our board of directors, including a vacancy resulting from an enlargement of our board of directors, may be filled only by the affirmative vote of a majority of our directors then in office, even if less than a quorum, or by a sole remaining director, subject to our stockholders agreement with respect to the director designation rights of the CD&R Allied Holdings, L.P., (the “CD&R Investor”). Any director elected to fill a vacancy will hold office until such director’s successor shall have been duly elected and qualified or until such director’s earlier death, resignation or removal. Our classified board of directors could have the effect of delaying or discouraging an acquisition of us or a change in our management.

Removal of Directors. Our amended and restated certificate of incorporation provides that directors may be removed with or without cause at any time upon the affirmative vote of holders of at least a majority of the outstanding shares of common stock then entitled to vote at an election of directors until the CD&R Investor ceases to beneficially own at least 40% of the outstanding shares of our common stock. Thereafter, our amended and restated certificate of incorporation provides that directors may be removed only for cause upon the affirmative vote of holders of at least a majority of the outstanding shares of common stock then entitled to vote at an election of directors.

Special Meetings of Stockholders. Our amended and restated certificate of incorporation provides that a special meeting of stockholders may be called only by the Chairman of our board of directors or by a resolution adopted by a majority of our board of directors. Special meetings may also be called by our corporate secretary at the request of the holders of at least a majority of the outstanding shares of our common stock until the CD&R Investor ceases to beneficially own at least 40% of the outstanding shares of our common stock. Thereafter, stockholders will not be permitted to call a special meeting of stockholders.

Stockholder Advance Notice Procedure. Our amended and restated by-laws provide for an advance notice procedure for stockholders to make nominations of candidates for election as directors or to bring other business before an annual meeting of our stockholders. Our amended and restated by-laws provide that any stockholder wishing to nominate persons for election as directors at, or bring other business before, an annual meeting must deliver to our corporate secretary a written notice of the stockholder’s intention to do so. These provisions may have the effect of precluding the conduct of certain business at a meeting if the proper procedures are not followed. We expect that these provisions may also discourage or deter a potential acquirer from conducting a solicitation of proxies to elect the acquirer’s own slate of directors or otherwise attempting to obtain control of our company. To be timely, the stockholder’s notice must be delivered to our corporate secretary at our principal executive offices not less than 90 days nor more than 120 days before the first anniversary date of the annual

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meeting for the preceding year; provided, however, that in the event that the annual meeting is set for a date that is more than 30 days before or more than 70 days after the first anniversary date of the preceding year’s annual meeting, a stockholder’s notice must be delivered to our corporate secretary (x) not less than 90 days nor more than 120 days prior to the meeting or (y) no later than the close of business on the 10th day following the day on which a public announcement of the date of the meeting is first made by us.

No Stockholder Action by Written Consent. Our amended and restated certificate of incorporation provides that stockholder action may be taken only at an annual meeting or special meeting of stockholders, provided that stockholder action may be taken by written consent in lieu of a meeting until the CD&R Investor ceases to beneficially own at least 40% of the outstanding shares of our common stock.

Amendments to Certificate of Incorporation and By-Laws. Our amended and restated certificate of incorporation provides that our amended and restated certificate of incorporation may be amended by both the affirmative vote of a majority of our board of directors and the affirmative vote of the holders of a majority of the outstanding shares of our common stock then entitled to vote at any annual or special meeting of stockholders; provided that, at any time when the CD&R Investor beneficially owns less than 40% of the outstanding shares of our common stock, specified provisions of our amended and restated certificate of incorporation may not be amended, altered or repealed unless the amendment is approved by the affirmative vote of the holders of at least 66 ²⁄₃% of the outstanding shares of our common stock then entitled to vote at any annual or special meeting of stockholders, including the provisions governing:

• liability and indemnification of directors;

• corporate opportunities;

• elimination of stockholder action by written consent if the CD&R Investor ceases to beneficially own at least 40% of the outstanding shares of our common stock;

• prohibition on the rights of stockholders to call a special meeting if the CD&R Investor ceases to beneficially own at least 40% of the outstanding shares of our common stock;

• removal of directors for cause if the CD&R Investor ceases to own at least 40% of our outstanding common stock;

• classified board of directors; and

• required approval of the holders of at least 66 ²⁄₃% of the outstanding shares of our common stock to amend our amended and restated by-laws and certain provisions of our amended and restated certificate of incorporation if the CD&R Investor ceases to beneficially own at least 40% of the outstanding shares of our common stock.

In addition, our amended and restated by-laws may be amended, altered or repealed, or new by-laws may be adopted, by the affirmative vote of a majority of the board of directors, or by the affirmative vote of the holders of (x) as long as the CD&R Investor beneficially owns at least 40% of the outstanding shares of our common stock, at least a majority, and (y) thereafter, at least 66 ²⁄₃%, of the outstanding shares of our common stock then entitled to vote at any annual or special meeting of stockholders.

These provisions make it more difficult for any person to remove or amend any provisions in our amended and restated certificate of incorporation and amended and restated by-laws that may have an anti-takeover effect.

Section 203 of the Delaware General Corporation Law. In our amended and restated certificate of incorporation, we have elected not to be governed by Section 203 of the DGCL, as permitted under and pursuant to subsection (b)(3) of Section 203, until the first date on which the CD&R Investor ceases to beneficially own (directly or indirectly) at least 5% of the outstanding shares of our common stock. Section 203 prohibits a publicly held Delaware corporation from engaging in a business combination, such as a merger, with a person or

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group owning 15% or more of the corporation’s outstanding voting stock for a period of three years following the date the person became an interested stockholder, unless (with certain exceptions) the business combination or the transaction in which the person became an interested stockholder is approved in a prescribed manner. Accordingly, we are not subject to any anti-takeover effects of Section 203.

Limitations on Liability and Indemnification

Our amended and restated certificate of incorporation contains provisions permitted under the DGCL relating to the liability of directors. These provisions eliminate a director’s personal liability for monetary damages resulting from a breach of fiduciary duty, except in circumstances involving:

• any breach of the director’s duty of loyalty;

• acts or omissions not in good faith or that involve intentional misconduct or a knowing violation of the law;

• Section 174 of the DGCL (unlawful dividends); or

• any transaction from which the director derives an improper personal benefit.

The principal effect of the limitation on liability provision is that a stockholder will be unable to prosecute an action for monetary damages against a director unless the stockholder can demonstrate a basis for liability for which indemnification is not available under the DGCL. These provisions, however, should not limit or eliminate our rights or any stockholder’s rights to seek non-monetary relief, such as an injunction or rescission, in the event of a breach of a director’s fiduciary duty. These provisions do not alter a director’s liability under federal securities laws. The inclusion of this provision in our amended and restated certificate of incorporation may discourage or deter stockholders or management from bringing a lawsuit against directors for a breach of their fiduciary duties, even though such an action, if successful, might otherwise have benefited us and our stockholders. In addition, your investment may be adversely affected to the extent we pay costs of settlement and damage awards against directors and officers pursuant to these indemnification provisions.

Our amended and restated certificate of incorporation and our amended and restated by-laws require us to indemnify and advance expenses to our directors and officers to the fullest extent not prohibited by the DGCL and other applicable law, except in the case of a proceeding instituted by the director without the approval of our board of directors. Our amended and restated certificate of incorporation and our amended and restated by-laws provide that we are required to indemnify our directors and officers, to the fullest extent permitted by law, for all judgments, fines, settlements, legal fees and other expenses incurred in connection with pending, threatened or completed legal proceedings because of the director’s or officer’s positions with us or another entity that the director or officer serves at our request, subject to various conditions, and to advance funds to our directors and officers to enable them to defend against such proceedings. To receive indemnification, the director or officer must have been successful in the legal proceeding or have acted in good faith and in what was reasonably believed to be a lawful manner in our best interest and, with respect to any criminal proceeding, have had no reasonable cause to believe his or her conduct was unlawful.

In connection with our initial public offering (our “IPO”), we entered into an indemnification agreement with each of our directors. The indemnification agreements provide our directors with contractual rights to the indemnification and expense advancement rights provided under our amended and restated by-laws, as well as contractual rights to additional indemnification as provided in the indemnification agreements. The indemnification agreements provide for the advancement or payment of all expenses to the indemnitee and for reimbursement to us if it is found that such indemnitee is not entitled to such indemnification under applicable law and our amended and restated certificate of incorporation and amended and restated by-laws.

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Corporate Opportunities

Our amended and restated certificate of incorporation provides that we, on our behalf and on behalf of our subsidiaries, renounce any interest or expectancy in, or in being offered an opportunity to participate in, corporate opportunities, that are from time to time presented to the CD&R Investor or any of its officers, directors, employees, agents, stockholders, members, partners, affiliates or subsidiaries (other than us and our subsidiaries), even if the opportunity is one that we or our subsidiaries might reasonably be deemed to have pursued or had the ability or desire to pursue if granted the opportunity to do so. Neither the CD&R Investor nor any of its officers, directors, employees, agents, stockholders, members, partners, affiliates or subsidiaries will generally be liable to us or any of our subsidiaries for breach of any fiduciary or other duty, as a director or otherwise, by reason of the fact that such person pursues or acquires such corporate opportunity, directs such corporate opportunity to another person or fails to present such corporate opportunity, or information regarding such corporate opportunity, to us or our subsidiaries unless, in the case of any such person who is a director or officer of Atkore, such corporate opportunity is expressly offered to such director or officer in writing solely in his or her capacity as a director or officer of Atkore. Stockholders will be deemed to have notice of and consented to this provision of our amended and restated certificate of incorporation.

Choice of Forum

Our amended and restated certificate of incorporation provides that, unless we consent in writing to the selection of an alternative forum, the Court of Chancery of the State of Delaware is, to the fullest extent permitted by law, the sole and exclusive forum for (i) any derivative action or proceeding brought on our behalf, (ii) any action asserting a claim of breach of a fiduciary duty owed to us or our stockholders by any of our directors, officers, other employees, agents or stockholders, (iii) any action asserting a claim arising out of or under the DGCL or as to which the DGCL confers jurisdiction on the Court of Chancery of the State of Delaware (including, without limitation, any action asserting a claim arising out of or pursuant to our amended and restated certificate of incorporation or our amended and restated by-laws) or (iv) any action asserting a claim that is governed by the internal affairs doctrine. By becoming a stockholder in our company, you will be deemed to have notice of and have consented to the provisions of our amended and restated certificate of incorporation related to choice of forum.

Market Listing

Our common stock is listed on the NYSE under the symbol “ATKR”.

Transfer Agent and Registrar

The transfer agent and registrar for our common stock is American Stock Transfer & Trust Company, LLC.

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SELLING STOCKHOLDER

As of November 17, 2017, CD&R Allied Holdings, L.P. beneficially owned 30,460,377 shares of the Company’s common stock. CD&R Allied Holdings, L.P. may offer and sell shares of our common stock under this prospectus and any accompanying prospectus supplement from time to time in amounts up to 30,460,377 shares of the Company’s common stock, at prices and on terms that will be determined at the time of the offering.

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PLAN OF DISTRIBUTION

General

The selling stockholder may sell the shares of our common stock covered by this prospectus using one or more of the following methods:

• underwriters in a public offering;

• “at the market” to or through market makers or into an existing market for the securities;

• ordinary brokerage transactions and transactions in which the broker-dealer solicits purchasers;

• block trades in which the broker-dealer will attempt to sell the securities as agent but may position and resell a portion of the block as principal to facilitate the transaction;

• purchases by a broker-dealer as principal and resale by the broker-dealer for its account;

• privately negotiated transactions;

• short sales (including short sales “against the box”);

• through the writing or settlement of standardized or over-the-counter options or other hedging or derivative transactions, whether through an options exchange or otherwise;

• by pledge to secure debts and other obligations;

• in other ways not involving market makers or established trading markets, including direct sales to purchasers or sales effected through agents;

• through the distribution by any selling stockholder to its partners, members or shareholders;

• a combination of any such methods; and

• any other method permitted pursuant to applicable law.

Registration of shares of our common stock covered by this prospectus does not mean that those securities necessarily will be offered or sold.

To the extent required by law, this prospectus may be amended or supplemented from time to time to describe a specific plan of distribution. Any prospectus supplement relating to a particular offering of our common stock by the selling stockholder may include the following information to the extent required by law:

• the name or names of the selling stockholder and the amounts to be sold by them;

• the terms of the offering;

• the names of any underwriters or agents;

• the purchase price of the securities;

• any delayed delivery arrangements;

• any underwriting discounts and other items constituting underwriters’ compensation;

• any initial public offering price; and

• any discounts or concessions allowed or reallowed or paid to dealers.

The selling stockholder may offer our common stock to the public through underwriting syndicates represented by managing underwriters or through underwriters without an underwriting syndicate. If underwriters are used for the sale of our common stock, the securities will be acquired by the underwriters for their own account. The underwriters may resell the common stock in one or more transactions, including in negotiated transactions at a fixed public offering price or at varying prices determined at the time of sale. In connection with any such underwritten sale of common stock, underwriters may receive compensation from the selling

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stockholder, for whom they may act as agents, in the form of discounts, concessions or commissions. Underwriters may sell common stock to or through dealers, and the dealers may receive compensation in the form of discounts, concessions or commissions from the underwriters and/or commissions from the purchasers for whom they may act as agents. Such compensation may be in excess of customary discounts, concessions or commissions.

If the selling stockholder uses an underwriter or underwriters to effectuate the sale of common stock, we and/or they will execute an underwriting agreement with those underwriters at the time of sale of those securities. To the extent required by law, the names of the underwriters will be set forth in the prospectus supplement used by the underwriters to sell those securities. Unless otherwise indicated in the prospectus supplement relating to a particular offering of common stock, the obligations of the underwriters to purchase the securities will be subject to customary conditions precedent and the underwriters will be obligated to purchase all of the securities offered if any of the securities are purchased.

In effecting sales, brokers or dealers engaged by the selling stockholder may arrange for other brokers or dealers to participate. Broker-dealers may receive discounts, concessions or commissions from the selling stockholder (or, if any broker-dealer acts as agent for the purchaser of shares, from the purchaser) in amounts to be negotiated. Such compensation may be in excess of customary discounts, concessions or commissions. If dealers are utilized in the sale of securities, the names of the dealers and the terms of the transaction will be set forth in a prospectus supplement, if required.

The selling stockholder may also sell shares of our common stock from time to time through agents. We will name any agent involved in the offer or sale of such shares and will list commissions payable to these agents in a prospectus supplement, if required. These agents will be acting on a best efforts basis to solicit purchases for the period of their appointment, unless we state otherwise in any required prospectus supplement.

The selling stockholder may sell shares of our common stock directly to purchasers. In this case, it may not engage underwriters or agents in the offer and sale of such shares.

Any underwriters, broker-dealers or agents that participate in the sale of the selling stockholder’s shares of common stock or interests therein may be “underwriters” within the meaning of the Securities Act of 1933, as amended (the “Securities Act”). Any discounts, commissions, concessions or profit they earn on any resale of the shares may be underwriting discounts and commissions under the Securities Act. Selling stockholders who are “underwriters” within the meaning of the Securities Act will be subject to the prospectus delivery requirements of the Securities Act. We will make copies of this prospectus available to the selling stockholder for the purpose of satisfying the prospectus delivery requirements of the Securities Act, if applicable. If any entity is deemed an underwriter or any amounts deemed underwriting discounts and commissions, the prospectus supplement will identify the underwriter or agent and describe the compensation received from the selling stockholder.

Certain of the underwriters, broker-dealers or agents who may become involved in the sale of the shares of common stock may engage in transactions with and perform other services for us in the ordinary course of their business for which they will receive ordinary compensation.

We are not aware of any plans, arrangements or understandings between the selling stockholder and any underwriter, broker-dealer or agent regarding the sale of the shares of our common stock by the selling stockholder. We cannot assure you that the selling stockholder will sell any or all of the shares of our common stock offered by them pursuant to this prospectus. In addition, we cannot assure you that the selling stockholder will not transfer, devise or gift the shares of our common stock by other means not described in this prospectus. Moreover, shares of common stock covered by this prospectus that qualify for sale pursuant to Rule 144 under the Securities Act may be sold under Rule 144 rather than pursuant to this prospectus.

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From time to time, the selling stockholder may pledge, hypothecate or grant a security interest in some or all of the shares it owns. The pledgees, secured parties or persons to whom the shares have been hypothecated will, upon foreclosure, be deemed to be the selling stockholder. The number of the selling stockholder’s shares offered under this prospectus will decrease as and when it takes such actions. The plan of distribution for that selling stockholder’s shares will otherwise remain unchanged. In addition, the selling stockholder may, from time to time, sell the shares short, and, in those instances, this prospectus may be delivered in connection with the short sales and the shares offered under this prospectus may be used to cover short sales.

The selling stockholder may enter into hedging transactions with broker-dealers and the broker-dealers may engage in short sales of the shares in the course of hedging the positions they assume with that selling stockholder, including, without limitation, in connection with distributions of the shares by those broker-dealers. The selling stockholder may enter into option or other transactions with broker-dealers that involve the delivery of the shares offered hereby to the broker-dealers, who may then resell or otherwise transfer those securities.

The selling stockholder may elect to make a pro rata in-kind distribution of the shares of common stock to its members, partners or shareholders. In such event, we may file a prospectus supplement to the extent required by law in order to permit the distributees to use the prospectus to resell the common stock acquired in the distribution.

Indemnification

We and the selling stockholder may enter agreements under which underwriters, dealers and agents who participate in the distribution of our common stock may be entitled to indemnification by us and/or the selling stockholder against various liabilities, including liabilities under the Securities Act, and to contribution with respect to payments which the underwriters, dealers or agents may be required to make.

Price Stabilization and Short Positions

If underwriters or dealers are used in the sale of shares of our common stock, until the distribution of the shares is completed, rules of the SEC may limit the ability of any underwriters to bid for and purchase our common stock. As an exception to these rules, representatives of any underwriters are permitted to engage in transactions that stabilize the price of our common stock. These transactions may consist of bids or purchases for the purpose of pegging, fixing or maintaining the price of our common stock. If the underwriters create a short position in shares of our common stock in connection with an offering (that is, if they sell more shares than are set forth on the cover page of the applicable prospectus supplement) the representatives of the underwriters may reduce that short position by purchasing shares in the open market.

We make no representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of our common stock. In addition, we make no representation that the representatives of any underwriters will engage in these transactions or that these transactions, once commenced, will not be discontinued without notice.

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U.S. FEDERAL INCOME TAX CONSIDERATIONS FOR NON-U.S. HOLDERS

The following is a discussion of certain U.S. federal income tax considerations relating to the purchase, ownership and disposition of our common stock by Non-U.S. Holders (as defined below) that purchase our common stock pursuant to this prospectus and any prospectus supplement and hold such common stock as a capital asset. This discussion is based on the Code, U.S. Treasury regulations promulgated or proposed thereunder, and administrative and judicial interpretations thereof, all as in effect on the date hereof and all of which are subject to change, possibly with retroactive effect, or to different interpretation. This discussion does not address all of the U.S. federal income tax considerations that may be relevant to specific Non-U.S. Holders in light of their particular circumstances or to Non-U.S. Holders subject to special treatment under U.S. federal income tax law (such as banks, insurance companies, dealers in securities or other Non-U.S. Holders that generally mark their securities to market for U.S. federal income tax purposes, foreign governments, international organizations, tax-exempt entities, certain former citizens or residents of the United States, or Non-U.S. Holders that hold our common stock as part of a straddle, hedge, conversion or other integrated transaction). This discussion does not address any U.S. state or local or non-U.S. tax considerations or any U.S. federal gift or alternative minimum tax considerations.

As used in this discussion, the term “Non-U.S. Holder” means a beneficial owner of our common stock that, for U.S. federal income tax purposes, is:

• an individual who is neither a citizen nor a resident of the United States;

• a corporation that is not created or organized in or under the laws of the United States, any state thereof or the District of Columbia;

• an estate that is not subject to U.S. federal income tax on income from non-U.S. sources that is not effectively connected with the conduct of a trade or business in the United States; or

• a trust unless (i) a court within the United States is able to exercise primary supervision over its administration and one or more U.S. persons have the authority to control all of its substantial decisions or (ii) it has in effect a valid election under applicable U.S. Treasury regulations to be treated as a U.S. person.

If an entity treated as a partnership for U.S. federal income tax purposes invests in our common stock, the U.S. federal income tax considerations relating to such investment will depend in part upon the status and activities of such entity and the particular partner. Any such entity should consult its own tax advisor regarding the U.S. federal income tax considerations applicable to it and its partners relating to the purchase, ownership and disposition of our common stock.

PERSONS CONSIDERING AN INVESTMENT IN OUR COMMON STOCK SHOULD CONSULT THEIR OWN TAX ADVISORS REGARDING THE U.S. FEDERAL, STATE AND LOCAL AND NON-U.S. INCOME, ESTATE AND OTHER TAX CONSIDERATIONS RELATING TO THE PURCHASE, OWNERSHIP AND DISPOSITION OF OUR COMMON STOCK IN LIGHT OF THEIR PARTICULAR CIRCUMSTANCES.

Distributions on Common Stock

If we make a distribution of cash or other property (other than certain pro rata distributions of our common stock or rights to acquire our common stock) with respect to a share of our common stock, the distribution generally will be treated as a dividend to the extent it is paid from our current or accumulated earnings and profits (as determined under U.S. federal income tax principles). If the amount of such distribution exceeds our current and accumulated earnings and profits, such excess generally will be treated first as a tax-free return of capital to the extent of the Non-U.S. Holder’s adjusted tax basis in such share of our common stock, and then as capital gain (which will be treated in the manner described below under “—Sale, Exchange or Other Disposition

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of Common Stock”). Distributions treated as dividends on our common stock that are paid to or for the account of a Non-U.S. Holder generally will be subject to U.S. federal withholding tax at a rate of 30%, or at a lower rate if provided by an applicable tax treaty and the Non-U.S. Holder provides the documentation (generally, Internal Revenue Service, or “IRS,” Form W-8BEN or W-8BEN-E) required to claim benefits under such tax treaty to the applicable withholding agent. Even if our current or accumulated earnings and profits are less than the amount of the distribution, the applicable withholding agent may elect to treat the entire distribution as a dividend for U.S. federal withholding tax purposes. Each Non-U.S. Holder should consult its own tax advisor regarding U.S. federal withholding tax on distributions, including such Non-U.S. Holder’s eligibility for a lower rate and the availability of a refund of any excess U.S. federal tax withheld.

If, however, a dividend is effectively connected with the conduct of a trade or business in the United States by a Non-U.S. Holder, such dividend generally will not be subject to the 30% U.S. federal withholding tax if such Non-U.S. Holder provides the appropriate documentation (generally, IRS Form W-8ECI) to the applicable withholding agent. Instead, such Non-U.S. Holder generally will be subject to U.S. federal income tax on such dividend in substantially the same manner as a U.S. person (except as provided by an applicable tax treaty). In addition, a Non-U.S. Holder that is treated as a corporation for U.S. federal income tax purposes may be subject to a branch profits tax at a rate of 30% (or a lower rate if provided by an applicable tax treaty) on its effectively connected income for the taxable year, subject to certain adjustments.

The foregoing discussion is subject to the discussion below under “—FATCA Withholding” and “—Information Reporting and Backup Withholding.”

Sale, Exchange or Other Disposition of Common Stock

A Non-U.S. Holder generally will not be subject to U.S. federal income tax on any gain recognized on the sale, exchange or other disposition of our common stock unless:

(i) such gain is effectively connected with the conduct of a trade or business in the United States by such Non-U.S. Holder, in which event such Non-U.S. Holder generally will be subject to U.S. federal income tax on such gain in substantially the same manner as a U.S. person (except as provided by an applicable tax treaty) and, if it is treated as a corporation for U.S. federal income tax purposes, may also be subject to a branch profits tax at a rate of 30% (or a lower rate if provided by an applicable tax treaty), subject to certain adjustments;

(ii) such Non-U.S. Holder is an individual who is present in the United States for 183 days or more during the taxable year of such sale, exchange or other disposition and certain other conditions are met, in which event such gain (net of certain U.S. source losses) generally will be subject to U.S. federal income tax at a rate of 30% (except as provided by an applicable tax treaty); or

(iii) we are or have been a “United States real property holding corporation” for U.S. federal income tax purposes at any time during the shorter of (x) the five-year period ending on the date of such sale, exchange or other disposition and (y) such Non-U.S. Holder’s holding period with respect to such common stock, and certain other conditions are met.

Generally, a corporation is a “United States real property holding corporation” if the fair market value of its United States real property interests equals or exceeds 50% of the sum of the fair market value of its worldwide real property interests and its other assets used or held for use in a trade or business (all as determined for U.S. federal income tax purposes). We believe that we presently are not, and we do not presently anticipate that we will become, a United States real property holding corporation. However, because this determination is made from time to time and is dependent upon a number of factors, some of which are beyond our control, including the value of our assets, there can be no assurance that we will not become a United States real property holding corporation. If we were a United States real property holding corporation during the period described in clause (iii) above, gain recognized by a Non-U.S. Holder generally would be treated as income effectively connected with the conduct of a trade or business in the United States by such Non-U.S. Holder, with the consequences

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described in clause (i) above (except that the branch profits tax would not apply), unless such Non-U.S. Holder owned (directly and constructively) five percent or less of our common stock during such period and our common stock is treated as “regularly traded on an established securities market” at any time during the calendar year of such sale, exchange or other disposition.

The foregoing discussion is subject to the discussion below under “—FATCA Withholding” and “—Information Reporting and Backup Withholding.”

FATCA Withholding

Under the Foreign Account Tax Compliance Act provisions of the Code and related U.S. Treasury guidance, or “FATCA,” a withholding tax of 30% will be imposed in certain circumstances on payments of (i) dividends on our common stock and (ii) on or after January 1, 2019, gross proceeds from the sale or other disposition of our common stock. In the case of payments made to a “foreign financial institution” (such as a bank, a broker, an investment fund or, in certain cases, a holding company), as a beneficial owner or as an intermediary, this tax generally will be imposed, subject to certain exceptions, unless such institution (i) has agreed to (and does) comply with the requirements of an agreement with the United States, or an “FFI Agreement,” or (ii) is required by (and does comply with) applicable foreign law enacted in connection with an intergovernmental agreement between the United States and a foreign jurisdiction, or an “IGA,” in either case to, among other things, collect and provide to the U.S. tax authorities or other relevant tax authorities certain information regarding U.S. account holders of such institution and, in either case, such institution provides the withholding agent with a certification as to its FATCA status. In the case of payments made to a foreign entity that is not a financial institution (as a beneficial owner), the tax generally will be imposed, subject to certain exceptions, unless such entity provides the withholding agent with a certification as to its FATCA status and, in certain cases, identifies any “substantial” U.S. owner (generally, any specified U.S. person that directly or indirectly owns more than a specified percentage of such entity). If our common stock is held through a foreign financial institution that has agreed to comply with the requirements of an FFI Agreement or is subject to similar requirements under applicable foreign law enacted in connection with an IGA, such foreign financial institution (or, in certain cases, a person paying amounts to such foreign financial institution) generally will be required, subject to certain exceptions, to withhold tax on payments made to (i) a person (including an individual) that fails to provide any required information or documentation or (ii) a foreign financial institution that has not agreed to comply with the requirements of an FFI Agreement and is not subject to similar requirements under applicable foreign law enacted in connection with an IGA. Each Non-U.S. Holder should consult its own tax advisor regarding the application of FATCA to the ownership and disposition of our common stock.

Information Reporting and Backup Withholding

Amounts treated as payments of dividends on our common stock paid to a Non-U.S. Holder and the amount of any U.S. federal tax withheld from such payments generally must be reported annually to the IRS and to such Non-U.S. Holder by the applicable withholding agent.

The information reporting and backup withholding rules that apply to payments of dividends to certain U.S. persons generally will not apply to payments of dividends on our common stock to a Non-U.S. Holder if such Non-U.S. Holder certifies under penalties of perjury that it is not a U.S. person (generally by providing an IRS Form W-8BEN or W-8BEN-E to the applicable withholding agent) or otherwise establishes an exemption.

Proceeds from the sale, exchange or other disposition of our common stock by a Non-U.S. Holder effected outside the United States through a non-U.S. office of a non-U.S. broker generally will not be subject to the information reporting and backup withholding rules that apply to payments to certain U.S. persons, provided that the proceeds are paid to the Non-U.S. Holder outside the United States. However, proceeds from the sale, exchange or other disposition of our common stock by a Non-U.S. Holder effected through a non-U.S. office of a non-U.S. broker with certain specified U.S. connections or a U.S. broker generally will be subject to these

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information reporting rules (but generally not to these backup withholding rules), even if the proceeds are paid to such Non-U.S. Holder outside the United States, unless such Non-U.S. Holder certifies under penalties of perjury that it is not a U.S. person (generally by providing an IRS Form W-8BEN or W-8BEN-E to the applicable withholding agent) or otherwise establishes an exemption. Proceeds from the sale, exchange or other disposition of our common stock by a Non-U.S. Holder effected through a U.S. office of a broker generally will be subject to these information reporting and backup withholding rules unless such Non-U.S. Holder certifies under penalties of perjury that it is not a U.S. person (generally by providing an IRS Form W-8BEN or W-8BEN-E to the applicable withholding agent) or otherwise establishes an exemption.

Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules generally will be allowed as a refund or a credit against a Non-U.S. Holder’s U.S. federal income tax liability if the required information is furnished by such Non-U.S. Holder on a timely basis to the IRS.

U.S. Federal Estate Tax

Shares of our common stock owned or treated as owned by an individual Non-U.S. Holder at the time of such Non-U.S. Holder’s death will be included in such Non-U.S. Holder’s gross estate for U.S. federal estate tax purposes and may be subject to U.S. federal estate tax unless an applicable estate tax treaty provides otherwise.

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LEGAL MATTERS

The validity of the shares of common stock offered in this offering will be passed upon for us by Debevoise & Plimpton LLP, New York, New York.

EXPERTS

The consolidated financial statements, and the related financial statement schedules, incorporated in this Prospectus by reference from the Company’s Annual Report on Form 10-K, and the effectiveness of Atkore International Group Inc.’s internal control over financial reporting have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their reports which are incorporated herein by reference. Such consolidated financial statements and financial statement schedules have been so incorporated in reliance upon the report of such firm given upon their authority as experts in accounting and auditing.

WHERE YOU CAN FIND MORE INFORMATION

This prospectus and any prospectus supplement are part of a registration statement that we filed with the SEC and do not contain all of the information set forth in the registration statement and the exhibits thereto because some parts have been omitted in accordance with the rules and regulations of the SEC. You will find additional information about us and the common stock being sold in this offering in the registration statement and the exhibits thereto. For further information with respect to Atkore and the common stock offered hereby, we refer you to the registration statement and the exhibits filed therewith. Statements contained in this prospectus and the documents incorporated by reference into this prospectus as to the contents of any contract or other document referred to are not necessarily complete and in each instance, if such contract or document is filed as an exhibit, reference is made to the copy of such contract or other document filed as an exhibit to the registration statement, each statement being qualified in all respects by such reference. A copy of the registration statement, including the exhibits thereto, may be read and copied at the SEC’s Public Reference Room at 100 F Street, N.E., Room 1580, Washington, D.C. 20549. Information on the operation of the Public Reference Room may be obtained by calling the SEC at 1-800-SEC-0330. In addition, the SEC maintains an internet site at http://www.sec.gov, from which interested persons can electronically access the registration statement, including the exhibits and any schedules thereto. Copies of the registration statement, including the exhibits and schedules thereto, are also available at your request, without charge, from Atkore International Group Inc., 16100 South Lathrop Avenue, Harvey, Illinois 60426, Attention: Legal Department.

We are subject to the informational requirements of the Exchange Act and, accordingly, file annual reports containing financial statements audited by an independent registered public accounting firm, quarterly reports containing unaudited financial statements, current reports, proxy statements and other information with the SEC. You may inspect and copy these reports, proxy statements and other information at the public reference facilities maintained by the SEC at the address noted above. You may also obtain copies of this material from the Public Reference Room of the SEC as described above, or inspect them without charge at the SEC’s website.

You may access, free of charge, our reports filed with the SEC (for example, our Annual Reports on Form 10-K, our Quarterly Reports on Form 10-Q and our Current Reports on Form 8-K and any amendments to those forms) through our website (http://investors.atkore.com). Reports filed with or furnished to the SEC will be available as soon as reasonably practicable after they are filed with or furnished to the SEC. None of the information contained on, or that may be accessed through, our websites or any other website identified herein is part of, or incorporated into, this prospectus. All website addresses in this prospectus are intended to be inactive textual references only.

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INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

The SEC allows us to “incorporate by reference” into this prospectus information we file with the SEC in other documents. This means that we can disclose important information to you by referring to another document we have filed with the SEC. The information relating to us contained in this prospectus should be read together with the information in the documents incorporated by reference.

We incorporate by reference, as of their respective dates of filing, the documents listed below (excluding any portions of such documents that have been “furnished” but not “filed” for purposes of the Exchange Act):

• our Annual Report on Form 10-K for the year ended September 30, 2017, filed with the SEC on November 29, 2017 (SEC File No. 001-37793);

• our Definitive Proxy Statement on Schedule 14A filed with the SEC on January 19, 2017, (only those parts incorporated in our Annual Report on Form 10-K for the year ended September 30, 2016).

• the description of capital stock contained in the Registration Statement on Form 8-A, as filed with the SEC on June 1, 2016 (SEC File No. 333-37793), as supplemented by the “Description of Capital Stock” found on page 6 of this prospectus and including any amendments or reports filed for the purpose of updating such description; and

• all documents filed by us with the SEC pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act, as amended (other than Current Reports on Form 8-K furnished under Items 2.02 and 7.01 (including any financial statements or exhibits relating thereto furnished pursuant to Item 9.01) of Form 8-K and not specifically incorporated by reference), after the date on which Post-Effective Amendment No. 2 to the registration statement that includes this prospectus was initially filed with the SEC and prior to the termination of the offerings to which this prospectus relates.

Any statement incorporated by reference in this prospectus from an earlier dated document that is inconsistent with a statement contained in this prospectus or in any other document filed after the date of the earlier dated document, but prior to the termination of the offerings to which this prospectus relates, which also is incorporated by reference into this prospectus, shall be deemed to be modified or superseded for purposes of this prospectus by such statement contained in this prospectus or in any other document filed after the date of the earlier dated document, but prior to the termination of the offerings to which this prospectus relates, which also is incorporated by reference into this prospectus.

Any person, including any beneficial owner, to whom this prospectus is delivered may request copies of this prospectus and any of the documents incorporated by reference into this prospectus, without charge, by written or oral request directed to Atkore International Group Inc., 16100 South Lathrop Avenue, Harvey, Illinois 60426, Attention: Investor Relations Dept., Telephone: (708) 339-1610 or the investor relations page of our website at http://investors.atkore.com or from the SEC through the SEC’s Internet website at the address provided under “Where You Can Find More Information”. All other information contained on our website is not a part of this prospectus. Documents incorporated by reference into this prospectus are available without charge, excluding any exhibits to those documents unless the exhibit is specifically incorporated by reference into those documents.

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7,234,838 Shares

Common Stock

PROSPECTUS SUPPLEMENT

Credit Suisse

May 16, 2018