UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

(Amendment No. 1)

For the Fiscal Year Ended December 31, 2013

or

For the transition period from                      to                     

Commission File Number 1-6249

WINTHROP REALTY TRUST

(Exact name of Registrant as specified in its certificate of incorporation)

(617) 570-4614

(Registrant’s telephone number, including area code)

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act: None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    Yes  ¨    No  x

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    Yes  ¨    No  x

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities and Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for at least the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§ 232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    Yes  x    No  ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer,” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check one):

Indicate by check mark whether the registrant is a shell company (as defined in Exchange Act Rule 12b-2).    Yes  ¨    No  x

As of March 1, 2014, there were 36,409,710 Common Shares outstanding.

At June 30, 2013, the aggregate market value of the Common Shares held by non-affiliates was $394,931,691.

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the registrant’s definitive Proxy Statement for the Annual Meeting of Shareholders, which is expected to be filed with the Securities and Exchange Commission within 120 days after the Registrant’s fiscal year ended December 31, 2013, are incorporated by reference into Part III hereof.

EXPLANATORY PARAGRAPH

This Form 10-K Amendment No.1 is being filed solely for the purpose of attaching as Exhibits 99.5 and 99.6 the audited financial statements of Vintage Housing Holdings LLC and 701 Seventh WRT Investor LLC and Subsidiaries as required by Rule 3-09 of Regulation S-X and to amend the Exhibit Index accordingly. This Form 10-K Amendment No. 1 has not been updated for events or information subsequent to the date of filing of the original Form 10-K for the year ended December 31, 2013 (the “Original 10-K”). Accordingly, this Form 10-K Amendment No. 1 should be read in conjunction with the Original 10-K and the registrant’s other filings with the Securities and Exchange Commission. Except as described above, no other changes have been made in this Amendment to modify or update the other disclosures presented in the Original 10-K.

ITEM 15 – EXHIBITS AND FINANCIAL STATEMENT SCHEDULES

(a) Financial Statements and Financial Statement Schedules.

(1) Financial Statements:

The following documents were filed as a part of the Original Filing:

Report of Independent Registered Public Accounting Firm.

Consolidated Balance Sheets - December 31, 2013 and 2012.

Consolidated Statements of Operations and Comprehensive Income - For the Years Ended December 31, 2013, 2012 and 2011.

Consolidated Statements of Shareholders’ Equity - For the Years Ended December 31, 2013, 2012 and 2011.

Consolidated Statements of Cash Flows - For the Years Ended December 31, 2013, 2012 and 2011.

Notes to Consolidated Financial Statements.

(2) Financial Statement Schedules:

The following financial statement schedules were filed as part of the Original Filing and should be read in conjunction with the Consolidated Financial Statements of the Registrant:

Schedule III - Real Estate and Accumulated Depreciation.

Schedule IV - Mortgage Loans on Real Estate.

All Schedules, other than III and IV, are omitted, as the information is not required or is otherwise furnished.

The Trust’s unconsolidated joint ventures, 701 Seventh WRT Investors LLC and Vintage Housing Holdings LLC, meet the definition of a significant subsidiary under S-X 210.1-02 (w) and financial statements for these entities are filed as part of this Form 10-K Amendment No. 1.

(b) Exhibits.

The exhibits listed on the Exhibit Index are filed as a part of this Report or incorporated by reference.

SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, as amended, Winthrop Realty Trust has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.