UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No.     )*

WINTHROP REALTY TRUST

(Name of Issuer)

Common Shares of Beneficial Interest

(Title of Class Securities)

976391300

(CUSIP Number)

September 25, 2014

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

¨ Rule 13d-1(b)

x Rule 13d-1(c)

¨ Rule 13d-1(d)

The information required in 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).

Item 1.

Winthrop Realty Trust

7 Bullfinch Place, Suite 500, Boston, Massachusetts 02114

This Schedule 13G is being filed jointly on behalf of:

(i) Indaba Capital Fund, L.P., a Cayman Islands exempted limited partnership (the “Fund”),

(ii) Indaba Capital Management, L.P., a Delaware limited partnership, and the Fund’s investment manager (the “Investment Manager”),

(iii) Indaba Partners, LLC, a Delaware limited liability company, and the Fund’s sole general partner (the “General Partner”),

(iv) IC GP, LLC, a Delaware limited liability company, and the Investment Manager’s sole general partner (“IC GP”), and

(v) Derek C. Schrier, a United States citizen and the Managing Member of IC GP and the Senior Managing Member of the General Partner (the “Senior Managing Member” and, collectively with the Investment Manager, the General Partner, IC GP and the Fund, the “Reporting Persons”).

The business address of each of the Investment Manager, the General Partner, IC GP and the Senior Managing Member is One Letterman Drive, Building D, Suite DM700, San Francisco, California 94129, USA. The registered office address of the Fund is c/o Maples Corporate Services Limited, PO Box 309, Ugland House, Grand Cayman KY1-1104, Cayman Islands.

The Investment Manager is a Delaware limited partnership, each of IC GP and the General Partner is a Delaware limited liability company and the Fund is a Cayman Islands exempted limited partnership. The Senior Managing Member is a United States citizen.

Common shares of beneficial interest

976391300

(a) Amount beneficially owned:

Incorporated by reference to Item 9 of the cover page pertaining to each Reporting Person.

The Reporting Persons specifically disclaim beneficial ownership in the shares of Common Shares reported herein except to the extent of their pecuniary interest therein.

Incorporated by reference to Item 11 of the cover page pertaining to each Reporting Person.

(c) Number of shares to which the person has:

(i) Sole power to vote or to direct the vote:

Incorporated by reference to Item 5 of the cover page pertaining to each Reporting Person.

(ii) Shared power to direct the vote:

Incorporated by reference to Item 6 of the cover page pertaining to each Reporting Person.

(iii) Sole power to dispose or to direct the disposition of:

Incorporated by reference to Item 7 of the cover page pertaining to each Reporting Person.

(iv) Shared power to dispose or to direct the disposition of:

Incorporated by reference to Item 8 of the cover page pertaining to each Reporting Person.

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class securities, check the following ¨.

Not applicable.

Not applicable.

Not applicable.

Not applicable.

By signing below, I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

SIGNATURE

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.

Date: October 3, 2014

EXHIBIT INDEX

Exhibit 1

JOINT FILING AGREEMENT

PURSUANT TO RULE 13D-1(k)1

The undersigned acknowledge and agree that the foregoing Schedule 13G is filed on behalf of each of the undersigned and that all subsequent amendments to this statement on Schedule 13G shall be filed on behalf of each of the undersigned without the necessity of filing additional joint filing agreements. The undersigned acknowledge that each shall be responsible for the timely filing of such amendments and for the completeness and accuracy of the information concerning him or it contained therein, but shall not be responsible for the completeness and accuracy of the information concerning the others, except to the extent that he or it knows or has reason to believe that such information is inaccurate.

Date: October 3, 2014