SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 17)*

CommonWealth REIT

(Name of Issuer)

Common Shares of Beneficial Interest, par value $0.01 per share

(Title of Class of Securities)

203233101

(CUSIP Number)

Keith Meister

Patrick J. Dooley, Esq.

Corvex Management LP

712 Fifth Avenue, 23rd Floor

New York, New York 10019

(212) 474-6700

Richard O’Toole

Related Fund Management, LLC

60 Columbus Circle

New York, New York 10023

(212) 421-5333

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

May 27, 2014

(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  ¨.

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 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).

This Amendment No. 17 to the Schedule 13D (this “Amendment No. 17”) relates to the common shares of beneficial interest, par value $0.01 per share (the “Shares”), of CommonWealth REIT, a Maryland real estate investment trust (the “Issuer” or “CommonWealth”) and amends the Schedule 13D filed on February 26, 2013, as amended by Amendment No. 1 thereto, filed with the SEC on February 27, 2013, Amendment No. 2 thereto, filed with the SEC on March 4, 2013, Amendment No. 3 thereto, filed with the SEC on March 4, 2013, Amendment No. 4 thereto, filed with the SEC on March 11, 2013, Amendment No. 5 thereto, filed with the SEC on March 13, 2013, Amendment No. 6 thereto, filed with the SEC on March 15, 2013, Amendment No. 7 thereto, filed with the SEC on March 28, 2013, Amendment No. 8 thereto, filed with the SEC on April 12, 2013, Amendment No. 9 thereto, filed with the SEC on April 18, 2013, Amendment No. 10 thereto, filed with the SEC on June 20, 2013, Amendment No. 11 thereto, filed with the SEC on June 24, 2013, Amendment No. 12 thereto, filed with the SEC on August 8, 2013, Amendment No. 13 thereto, filed with the SEC on November 19, 2013, Amendment No. 14 thereto, filed with the SEC on November 25, 2013, Amendment No. 15 thereto, filed with the SEC on February 12, 2014 and Amendment No. 16 thereto, filed with the SEC on March 27, 2014 (the “Original Schedule 13D” and, together with this Amendment No. 17, the “Schedule 13D”). Capitalized terms used and not defined in this Amendment No. 17 have the meanings set forth in the Original Schedule 13D.

This Amendment No. 17 is being filed by (i) Corvex Management LP, a Delaware limited partnership (“Corvex”), and Keith Meister, (ii) Related Fund Management, LLC, a Delaware limited liability company (“Related”), Related Real Estate Recovery Fund GP-A, LLC, a Delaware limited liability company, Related Real Estate Recovery Fund GP, L.P., a Delaware limited partnership, Related Real Estate Recovery Fund, L.P., a Delaware limited partnership (“Related Recovery Fund”), and RRERF Acquisition, LLC, a Delaware limited liability company (“RRERF”) and (iii) David R. Johnson (the “Individual Shareholder”). As set forth below, as of the date of the Individual Shareholder Termination (as defined herein), the Corvex Persons and the Related Persons shall no longer be deemed to be a “group” for purposes of Section 13(d)(3) of the Exchange Act and Rule 13d-5(b) promulgated thereunder with the Individual Shareholder, such that the filing of this Amendment No. 17 represents an exit filing for the Individual Shareholder only.

This Amendment No. 17 is being filed to amend Item 4, Item 5, Item 6 and Item 7 of the Schedule 13D as follows:

Item 4 Purpose of Transaction

Item 4 of the Schedule 13D is amended by adding the following:

On May 23, 2014, at a special meeting of the shareholders of the Issuer, each of Corvex and Related’s nominees – James Corl, Edward Glickman, David Helfand, Peter Linneman, Jim Lozier, Kenneth Shea and Samuel Zell – were elected to the Board of Trustees of the Issuer (the “Election”). Following the Election, on May 23, 2014 Corvex and Related issued a press release. The Press Release is attached as Exhibit 35 hereto and is incorporated by reference in this Item 4 in its entirety.

On May 27, 2014, in accordance with the EGI Agreement, previously filed as Exhibit 32 to the Schedule 13D, Corvex and Related Recovery Fund each delivered separate undertakings to the Board of Trustees of the Issuer. A form of the undertaking is attached as Exhibit 36 hereto and is incorporated by reference in this Item 4 in its entirety. In accordance with its terms, the EGI Agreement terminated on May 27, 2014, one business day following the Election, provided, that (a) the Options, as described in Section 3 thereof, continue in effect and can be exercised until July 22, 2014 and (b) Section 8 (Indemnification) thereof also survived termination. As such, the Corvex Persons and the Related Persons may no longer be deemed to be a “group” for purposes of Section 13(d)(3) of the Exchange Act and Rule 13d-5(b) promulgated thereunder with the EGI Persons.

On May 28, 2014, Corvex and Related Recovery Fund terminated the Support Agreement with the Individual Shareholder, dated April 11, 2013 (the “Individual Shareholder Termination”). A form of the Support Agreement was previously filed as Exhibit 18 to the Schedule 13D and provided that the Individual Shareholder would cooperate with Corvex and Related Recovery Fund in certain activities regarding the Issuer. The Individual Shareholder Termination is attached as Exhibit 37 hereto and is incorporated by reference in this Item 4 in its entirety.

Effective upon the execution of the Individual Shareholder Termination the Corvex Persons and the Related Persons shall no longer be deemed to beneficially own the Additional Shares (as defined herein). The Corvex Persons and the Related Persons may no longer be deemed to be a “group” for purposes of Section 13(d)(3) of the Exchange Act and Rule 13d-5(b) promulgated thereunder with the Individual Shareholder.

Item 5 Interest in Securities of the Issuer

Item 5 of the Schedule 13D is amended by adding the following:

The percentages set forth in this Schedule 13D are calculated based upon an aggregate of 128,966,467 Shares outstanding, comprised of (i) 10,530,052 Shares which the Issuer expected to issue in order to effect the conversion of the 6 ¹/₂% Series D Cumulative Convertible Preferred Shares, as reported in the Issuer’s Current Report on Form 8-K filed with the SEC on May 15, 2014 and (ii) 118,436,415 Shares outstanding as of May 7, 2014, as reported in the Issuer’s Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2014, filed with the SEC on May 8, 2014.

(a)-(b) Each of the Corvex Persons may be deemed to be the beneficial owner of 11,350,500 Shares (representing approximately 8.8% of the Issuer’s outstanding Shares) which include: (i) 5,675,250 Shares held on behalf of the Corvex Funds (the “Corvex Shares”) and (ii) 5,675,250 held on behalf of RRERF (the “Related Shares”). By virtue of his position as a control person of the general partner of Corvex, Mr. Meister and Corvex may be deemed to share voting power and dispositive power with respect to the Corvex Shares. In addition, by virtue of the Agreement, the Corvex Persons may be deemed to share with the Related Persons voting power and dispositive power with respect to the Related Shares. Each of the Corvex Persons disclaims beneficial ownership with respect to the Related Shares.

Each of the Related Persons may be deemed to be the beneficial owner of 11,350,500 Shares (representing approximately 8.8% of the Issuer’s outstanding Shares), which include: (i) the Related Shares and (ii) the Corvex Shares. By virtue of their relationship, the Related Persons may be deemed to share voting power and dispositive power with respect to the Related Shares. In addition by virtue of the Agreement, the Related Persons may be deemed to share with the Corvex Persons voting power and dispositive power with respect to the Corvex Shares. Each of the Related Persons disclaims beneficial ownership with respect to the Corvex Shares.

The Individual Shareholder beneficially owns 10,410.0658 Shares (the “Additional Shares”) (representing less than 1% of the Issuer’s outstanding Shares) which number includes 1,440 Shares received upon the conversion of the Individual Shareholder’s 1,423.4834 shares of the Issuer’s 6 ¹/₂% Series D Cumulative Convertible Preferred Shares.

(c) Except as set forth below, there have been no transactions with respect to the Shares during the sixty days prior to the date of filing of this Schedule 13D by any of the Reporting Persons or, to their knowledge, any other person or entity referred to in Item 2 of the Schedule 13D.

On May 14, 2014, the Fundamental Change Conversion Date, after exercising the Fundamental Change Conversion Right, the Individual Shareholder’s 1,423.4834 6 ¹/₂% Series D Cumulative Convertible Preferred Shares were converted by the Issuer into 1,440 Shares.

The terms “Fundamental Change Conversion Date” and “Fundamental Change Conversion Right” have the definitions assigned to them in the Articles Supplementary dated October 10, 2006 establishing the terms of the 6 ¹/₂% Series D Cumulative Convertible Preferred Shares.

(d) Except as set forth below, no person is known to have the right to receive, or the power to direct the receipt of dividends from, or proceeds from the sale of, the Shares beneficially owned by any of the Reporting Persons, other than the Reporting Persons themselves and investment funds, institutions and mutual funds for which some of the Reporting Persons provide management services.

The limited partners of (or investors in) each of the private investment funds, or their respective subsidiaries or affiliated entities, for which Corvex or its affiliates acts as general partner and/or investment adviser have the right to participate in the receipt of dividends from, or proceeds from the sale of, the Shares held for the accounts of their respective funds in accordance with their respective limited partnership interests (or investment percentages) in their respective funds.

The limited partners of Related Recovery Fund have the right to participate in the receipt of certain dividends and proceeds from the sale of the Shares, in each case in accordance with their respective limited partnership interests.

(e) Not Applicable

Item 6 Contracts, Arrangements, Understandings, or Relationships with Respect to the Securities of the Issuer

Item 6 of the Schedule 13D is amended by adding the following:

The information contained in Item 4 of this Amendment No. 17 is incorporated herein by reference.

Item 7 Material to be Filed as Exhibits

Item 7 of the Schedule 13D is amended by adding thereto the following:

SIGNATURES

After reasonable inquiry and to the best of our knowledge and belief, the undersigned certifies that the information set forth in this statement is true, complete and correct.

Exhibit 35

CORVEX AND RELATED ANNOUNCE RESULTS OF COMMONWEALTH REIT’S SPECIAL MEETING OF SHAREHOLDERS

Entire Slate of Trustees Nominated by Related and Corvex Elected to Board

NEW YORK, May 23, 2014 – Corvex Management LP (“Corvex”) and Related Fund Management, LLC (“Related”), whose separately managed investment funds collectively own approximately 8.8% of the outstanding shares of CommonWealth REIT (NYSE:CWH), today announced that their entire slate of nominees, James Corl, Edward Glickman, David Helfand, Peter Linneman, Jim Lozier, Kenneth Shea and Samuel Zell, were today elected to serve on the board of CommonWealth REIT at a special meeting of shareholders. The slate was elected by approximately 85% of the shares outstanding as of the record date and is the culmination of Corvex and Related’s efforts to allow shareholders to exercise their rights, regain control of CommonWealth REIT and realize value for all shareholders.

Keith Meister of Corvex and Jeff T. Blau of Related, said:

“Today is a historic milestone for CommonWealth and the REIT industry, and a tremendous victory for shareholder rights. The election of a slate of truly independent and accountable trustees who are committed to the future success of CommonWealth for the long-term benefit of all shareholders finally puts an end to a long-fought battle for shareholder democracy and perennial underperformance at CommonWealth.

“From the very beginning, our sole focus was allowing all shareholders to exercise their rights, offering the ability to effect change and restore shareholder value. We are extremely gratified by the strong and broad support we consistently received from retail and institutional shareholders. We remain confident that the highly qualified and experienced board elected today will work expeditiously and successfully to usher in a new era of accountability that fosters the incentives critical in building a successful company focused on the long-term creation of shareholder value.”

Corvex and Related first announced the acquisition of their interests in CommonWealth REIT in February of 2013, releasing a detailed plan to effect change and realize value for all shareholders. Among other things, Corvex and Related called for a fair and unfettered election of a new Board consisting solely of truly independent trustees; corporate governance reform; and the internalization of management and alignment of compensation with shareholder returns. Despite repeated obstruction and litigation efforts by the previous board to eliminate shareholder rights, a consent solicitation was launched in February of 2014 and the entire board of directors was removed by a vote of over 81% of outstanding shares of CWH in March.

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About Corvex Management LP

Corvex Management LP is an investment firm headquartered in New York, New York that engages in value-based investing across the capital structure in situations with identifiable catalysts. Corvex was founded in March 2011 and follows an opportunistic approach to investing with a specific focus on equity investments, special situations and distressed securities largely in North America.

About Related Fund Management LLC

Related Fund Management, LLC is an affiliate of Related Companies, one of the most prominent privately-owned real estate firms in the United States. Formed 40 years ago, Related is a fully integrated, highly diversified industry leader with experience in virtually every aspect of development, acquisitions, management, finance, marketing and sales. Related’s existing portfolio of real estate assets, valued at over $15 billion, is made up of best-in-class mixed-use, residential, retail, office and affordable properties. For more information about Related Companies please visit www.related.com.

For further information, contact:

Rupal Doshi

Corvex

(212) 474-6750

rdoshi@corvexcap.com

Joanna Rose

Related

(212) 801-3902

jrose@related.com

Exhibit 36

The Board of Trustees

CommonWealth REIT

Two North Riverside Plaza, Suite 600

Chicago, IL 60606

Gentlemen,

The undersigned hereby undertakes that, from the date hereof until May 23, 2015, it shall cause with respect to common shares of beneficial interest of CommonWealth REIT (the “Company”) it or any of its Covered Entities beneficially owns, (a) as of the record date for each meeting of shareholders of the Company, to be present for quorum purposes and to be voted at each such shareholder meeting or at any adjournments or postponements thereof, in favor of all actions recommended by the Board of Trustees of the Company (the “Board”) in its definitive proxy statement for such shareholder meeting; and (b) as of the record date for any proposed action by written consent, to consent to all actions recommended by the Board in its definitive consent solicitation statement and to take no action with respect to shareholder proposals not recommended by the Board.

As used herein, (1) the term “Covered Entities” shall mean each of the undersigned’s controlled affiliates, and any of its and its controlled affiliates’ respective investment funds, managed accounts or other investment vehicles managed or advised by them, provided, however, that in no event shall any person be deemed to be a Covered Entity solely because another person serves as a director, trustee or officer of such person and (2) “affiliate” with respect to any person, means any other person who is an “affiliate” of that person within the meaning of Rule 405 promulgated under the Securities Act of 1933, as amended.

This undertaking is delivered to, and solely for the benefit of, the Board in its capacity as such, and shall be governed by and construed in accordance with the laws of the State of New York.

Sincerely,

Exhibit 37

May 28, 2014

David R. Johnson

206 Island View Lane

Seneca, SC 29672

Dear Mr. Johnson,

Reference is made to that certain Support Agreement by and among Corvex Management LP (“Corvex”), Related Real Estate Recovery Fund, L.P. (“Related”) and yourself, dated as of April 11, 2013 (the “Support Agreement”). In accordance with the terms of the Support Agreement, Corvex and Related hereby deliver this written notice terminating the Support Agreement, effective immediately.

We would like to express our sincere appreciation for your support in our endeavors.