UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

(Amendment No. 1)

For the fiscal year ended December 31, 2015

or

For the transition period from . to .

Commission File No. 1-35933

GRAMERCY PROPERTY TRUST

(Exact name of registrant as specified in its charter)

Chambers Street Properties

47 Hulfish Street, Suite 210, Princeton, New Jersey 08542

(Former name, former address and former fiscal year, if changed since last report)

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 x      No ¨

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 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 the past 90 days. Yes x      No ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Website, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (Section 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 the 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 small reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “small reporting company” in Rule 12b-2 of the Exchange Act.

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

As of February 25, 2016, the Registrant had 421,011,239 common shares outstanding. The aggregate market value of common shares held by non-affiliates of the registrant (235,498,633 shares) at June 30, 2015, was $1,872,214,132. The aggregate market value was calculated by using the closing price of the common shares as of that date on the New York Stock Exchange, which was $7.95 per share.

EXPLANATORY NOTE

We are filing this Amendment No. 1 on Form 10-K/A (the “Amendment”) to our Annual Report on Form 10-K for the fiscal year ended December 31, 2015, filed with the Securities and Exchange Commission on February 29, 2016 (the “Original Filing”), for the sole purpose of filing Exhibit 12.1 and 21.1, which were erroneously omitted from the Original Filing.

This amendment speaks as of the filing date of the Original Filing, does not reflect events occurring after the original filing date or modify or update those disclosures that may be affected by subsequent events, and no other changes are being made to any other disclosure contained in the Original Filing or any exhibits thereto.

PART IV

(a) The following documents are filed as a part of this Report:

SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized, on March 2, 2016.