UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d)

OF THE SECURITIES EXCHANGE ACT OF 1934

Date of report (Date of earliest event reported): August 13, 2013

RXi PHARMACEUTICALS

CORPORATION

(Exact name of registrant as specified in its charter)

1500 West Park Drive, Suite 210

Westborough, Massachusetts 01581

(Address of Principal Executive

Offices) (Zip Code)

Registrant’s telephone number, including area code: (508) 767-3861

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

On August 14, 2013, RXi Pharmaceuticals Corporation (the “Company”) reported its results of operations for the quarter ended June 30, 2013. A copy of the press release is furnished as Exhibit 99.1 to this Current Report on Form 8-K (the “Report”).

The information in this Item 2.02 and attached as Exhibit 99.1 to this Report will not be treated as “filed” for the purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liabilities of that section. This information will not be incorporated by reference into any filing under the Securities Act of 1933, as amended, or into another filing under the Exchange Act, unless that filing expressly incorporates this information by reference.

On August 13, 2013, the Company entered into an exchange agreement (the “Exchange Agreement”) with Tang Capital Partners, L.P. (“TCP”). In connection with the Exchange Agreement, the Company designated a new series of preferred stock on August 13, 2013: its Series A-1 Convertible Preferred Stock, par value $0.0001 (the “Series A-1 Preferred Stock”). Pursuant to the Exchange Agreement, TCP exchanged a total of 2,000 shares of Series A Preferred Stock for a like number of shares of Series A-1 Preferred Stock (the “Series A-1 Shares”). The terms of the Series A-1 Preferred Stock are identical in all respects to the Series A Preferred Stock, other than the elimination of cash penalties that would potentially be due and payable upon the failure of the Company to have enough shares of its Common Stock available to permit the conversion of Series A Preferred Stock into Common Stock.

Neither the Series A-1 Shares nor the issuance of such Series A-1 Shares were registered under the Securities Act of 1933, as amended (the “Securities Act”). The Company issued the Series A-1 Shares in a transaction exempt from the registration requirements of the Securities Act by virtue of the exemption provided for in Section 3(a)(9) of the Act for securities exchanged by the issuer with an existing security holder. No commission or other remuneration was paid or given directly or indirectly for soliciting such exchange.

The information set forth under Item 3.03 above is incorporated herein by reference.

*    *    *

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.