UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 5)*

First Interstate BancSystem, Inc.

(Name of Issuer)

Class A Common Stock and Class B Common Stock

(Title of Class of Securities)

Class A Common Stock: 32055Y 201; Class B Common Stock: 32055Y 300

(CUSIP Numbers)

Thomas W. Scott

c/o First Interstate BancSystem, Inc.

401 North 31st Street

Billings, Montana 59116

(406) 255-5390

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

December 31, 2015

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  x

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See §240.13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

(Page 1 of 19 Pages)

This filing relates to shares of the Class A common stock (“Class A Stock”), no par value per share, and the Class B common stock (“Class B Stock”), no par value per share (collectively, the “Common Stock”), of First Interstate BancSystem, Inc., a Montana corporation (the “Issuer”), which has its principal executive offices at 401 North 31st Street, Billings, Montana 59116. The Class A Stock is listed on the NASDAQ stock market under the symbol “FIBK.” The Class B Stock, which is not listed on any market or exchange, is convertible at any time into Class A Stock on a share for share basis at the discretion of the holder. The conversion feature of the Class B Stock does not expire. Each share of Class A Stock is entitled to one vote per share. Each share of Class B Stock is entitled to five votes per share.

The Issuer had 21,698,594 shares of Class A Stock and 23,729,631 shares of Class B Stock outstanding as of December 31, 2015. All beneficial ownership and voting power percentage calculations with respect to the Common Stock are based on the outstanding shares as of December 31, 2015.

All beneficial ownership calculations contained in this Schedule 13D have been made in accordance with Rule 13d-3 of the Securities Exchange Act of 1934, as amended.

(a) – (c) This schedule is being filed by each of Randall I. Scott, N Bar 5, Limited Partnership (“N Bar 5”), James R. Scott, J.S. Investments Limited Partnership (“J.S. Investments”), John M. Heyneman, Jr., Towanda Investments Limited Partnership (“Towanda”), Thomas W. Scott, and Homer A. Scott, Jr.(each, a “Reporting Person” and collectively, the “Reporting Persons”).

As of December 31, 2015, the Reporting Persons as a group are the beneficial owners of 16,780,972 shares of Common Stock (of which 16,654,212 are shares of Class B Stock and 126,760 are shares of Class A Stock). Such shares of Common Stock represent beneficial ownership of 43.8% of Class A Stock and 70.1% of Class B Stock. Such shares of Common Stock represent 59.4% of the voting power of the Common Stock.

Mr. Randall I. Scott has been a director of the Issuer since May 2012 and was previously a director of the Issuer from 1993 to 2002 and from 2003 to 2011. He is a certified financial planner and managing general partner of N Bar 5. N Bar 5 is a Delaware limited partnership engaged in the business of managing investments. The business address for each of Mr. Scott and N Bar 5 is P.O. Box 7113, Billings, Montana 59103.

Mr. James R. Scott has been a director of the Issuer since 1971, the Vice Chairman of the Issuer’s board of directors from 1990 through 2012 and the Executive Vice Chairman of the Issuer’s board of directors since 2012. On January 21, 2016, Mr. Scott assumed the role of Chairman of the Company’s board of directors. Mr. Scott is also Chairman of the Padlock Ranch Corporation, Managing Partner of J.S. Investments and a board member of Foundation for Community Vitality. J.S. Investments is a Delaware limited partnership engaged in the business of managing investments. The business address for each of Mr. Scott and J.S. Investments is P.O. Box 7113, Billings, Montana 59103.

Mr. John M. Heyneman, Jr. has been a director of the Issuer since 2010. Mr. Heyneman was previously Project Manager of Partnership for Wyoming’s Future from 2009 through 2012. Mr. Heyneman is Managing General Partner of Towanda, a Delaware limited partnership engaged in the business of managing investments. The business address for each of Mr. Heyneman and Towanda is 4100 Big Horn Avenue, Sheridan, Wyoming 82801.

Mr. Thomas W. Scott has been a director of the Issuer since 1971 and the Chairman of the Issuer’s board of directors since 2004. Mr. Scott retired from the Company’s board of directors effective January 21, 2016. Mr. Scott’s business address is P.O. Box 30918, Billings, Montana 59116.

Mr. Homer A. Scott, Jr. is a retired director of the Issuer. Mr. Scott’s business address is P.O. Box 2007, Sheridan, Wyoming 82801.

(d) – (e) During the last five years, none of the foregoing persons has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors) or been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

(f) Each of the foregoing Reporting Persons and other natural persons for whom information has been provided is a citizen of the United States of America.

This filing does not reflect any purchase of securities by the Reporting Persons. Rather, this filing is being made due to the combination of the Reporting Persons for the purpose of acting together as a group as set forth below in Item 4.

Each Reporting Person, as either one of the descendents of Homer Scott, Sr., the founder of the Issuer, or an affiliate thereof, has been a long-time beneficial owner of shares of Common Stock. The shares of capital stock held by each of the Reporting Persons have been held without any arrangement, relationship, understanding or agreement among them to act together for the purpose of acquiring, holding, voting or disposing of any of such equity securities or to otherwise seek to influence on a concerted basis control over the Issuer, other than the Shareholder Agreement (as defined below), including predecessor versions thereof, which contains certain customary transfer restrictions and provides the parties thereto a right of first refusal, except for certain permitted transfers.

The Issuer completed an initial public offering of Class A Stock and related listing of such stock on The NASDAQ Stock Market LLC in March 2010. The Reporting Persons, which own collectively and in the aggregate more than 50% of the outstanding voting securities of the Issuer, reached an oral agreement as a group, effective as of the date of the Joint Filing Agreement included as an exhibit hereto, for the purpose of influencing control over the Issuer solely to the extent necessary to ensure the Issuer will qualify as a “controlled company” under applicable NASDAQ Marketplace Rules. Under such rules, NASDAQ listed issuers that qualify as controlled companies may avail themselves of certain corporate governance exemptions involving board and committee independent director requirements. Except for the foregoing and the Shareholder Agreement, the Reporting Persons do not have any arrangement, relationship, understanding or agreement among them to act together for the purpose of acquiring, holding, voting or disposing of any of such equity securities or to otherwise seek to influence control over the Issuer.

Other than in connection with the contracts, arrangements, understandings or relationships with respect to securities of the Issuer described in Item 6 below, including the Scott Family Shareholders’ Agreement among Scott family members dated October 29, 2010 (the “Shareholder Agreement”), there are no other plans or proposals that would result in the acquisition of additional securities or disposition of additional securities of the Issuer.

Except as described above in this Item 4, the Reporting Persons do not have, as of the date hereof, any other plans or proposals that relate to or would result in any of the actions or events specified in clauses (a) through (j) of Item 4 of Schedule 13D. Each of the Reporting Persons reserves the right, however, to formulate such plans or proposals, and to take such action with respect to any or all of such matters and any other matters as they may determine in their sole discretion.

(a) – (b)

The group comprised of the Reporting Persons included in this filing beneficially owns an aggregate of 16,780,972 shares (of which 16,654,212 are shares of Class B Stock and 126,760 are shares of Class A Stock), representing 43.8% of the Class A Stock and 70.1% of the Class B Stock. Such shares of Common Stock represent 59.4% of the voting power of the Common Stock. This amount includes shares beneficially owned as of the date hereof by each Reporting Person as set forth below.

(c) The following transactions in shares of Common Stock of the Issuer during the 60 days preceding the date hereof were effected by persons identified in Item 2. Shares of Class B Stock transferred to persons other than “permitted transferees” under the Issuer’s Amended and Restated Articles of Incorporation are automatically converted into shares of Class A Stock upon transfer.

(d) The Reporting Persons know of no person that has the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of securities of the Issuer.

(e) Not applicable.

ITEM 6. CONTRACTS, ARRANGEMENTS, UNDERSTANDINGS OR RELATIONSHIPS WITH RESPECT TO SECURITIES OF THE ISSUER

As described in Item 4 above, the Reporting Persons have agreed orally to effect as a group influence or control over the Issuer. As referenced in Item 4 above, the Reporting Persons are parties to or subject to the contracts, arrangements, understandings, and relationships described in this Item 6.

Currently, the Reporting Persons are parties to the Shareholder Agreement which contains certain customary transfer restrictions with respect to the Class B Stock and provides the parties thereto a right of first refusal, except for certain permitted transfers. The primary purpose of the Shareholder Agreement is to preserve opportunities within the extended Scott family to purchase shares of Class B Stock before they are converted and sold in the open market as shares of Class A Stock. The Shareholder Agreement is not applicable to shares of Class A Stock and, except as specifically set forth therein, does not provide for any arrangement, relationship, understanding or agreement among them to act together for the purpose of acquiring, holding, voting or disposing of any of such equity securities or to otherwise seek to influence on a concerted basis control over the Issuer. A copy of the Shareholder Agreement was previously filed with the Securities and Exchange Commission. The foregoing description of the Shareholder Agreement does not purport to be complete and is qualified in its entirety by reference to the full text thereof, which is incorporated herein by reference.

Additionally, certain of the Reporting Persons have pledged their interests in the shares of Common Stock as collateral for loans from third-party financial institutions pursuant to various loan agreements.

* Previously filed.

SIGNATURES

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

EXHIBIT INDEX

* Previously filed.