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): May 8, 2013 (May 6, 2013)

GENSPERA, INC.

(Exact name of registrant as specified in Charter)

2511 N Loop 1604 W, Suite 204

San Antonio, TX 78258

(Address of Principal Executive Offices)

210-479-8112

(Issuer Telephone number)

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):

¨ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

¨ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

¨ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

¨ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Resignation of Principal Accounting Officer

On May 6, 2013, GenSpera, Inc. (“Company”) received the resignation of Nancy Jean Barnabei as Vice President Finance, Treasurer and Principal Accounting Officer. Ms. Barnabei’s final date of employment has yet to be determined but is anticipated to be no later than June 30, 2013. The resignation was not as a result of any disagreement with the Company, known to an executive officer, relating to the Company’s operations, policies or practices.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this Report on Form 8-K to be signed on its behalf by the undersigned hereunto duly authorized.

Dated: May 8, 2013