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): November 5, 2015

Mast Therapeutics, Inc.

(Exact name of Registrant as Specified in Its Charter)

Registrant’s Telephone Number, Including Area Code: (858) 552-0866

Not Applicable

(Former Name or Former Address, if Changed Since Last Report)

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 Instructions A.2. below):

Item 5.02 Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers.

On November 5, 2015, the board of directors (the “Board”) of Mast Therapeutics, Inc. (the “Company”) elected Peter Greenleaf to the Board, effective immediately.  Mr. Greenleaf has not been appointed to any committee of the Board.  The Board determined that Mr. Greenleaf is an “independent director,” as such term is defined in Section 803(A)(2) of the NYSE MKT LLC Company Guide.

As a non-employee member of the Board, pursuant to the Company’s current non-employee director compensation policy, which was adopted in December 2014, Mr. Greenleaf will receive an annual cash retainer of $35,000, payable in equal quarterly installments and pro-rated with respect to the fourth quarter of 2015. In addition, pursuant to the non-employee director compensation policy, on November 5, 2015, the Board granted Mr. Greenleaf an “inducement option” to purchase up to 64,791 shares of the Company’s common stock and a second option to purchase up to 37,794 shares of the Company’s common stock, which were granted under the Company’s 2015 Omnibus Incentive Plan. The option awards are otherwise subject to the terms and conditions set forth in the non-employee director compensation policy as described in the Company’s definitive proxy statement for the 2015 Annual Meeting of Stockholders held on June 11, 2015, filed with the Securities and Exchange Commission on April 29, 2015.

The Company also has entered into an Indemnification Agreement with Mr. Greenleaf. The Indemnification Agreement provides for indemnification and advancement of litigation and other expenses to Mr. Greenleaf to the fullest extent permitted by law for claims relating to his service to the Company or its subsidiaries. The Indemnification Agreement is identical in all material respects to the indemnification agreements entered into with the Company’s other directors and officers.

2

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.

3