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): June 17, 2015

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))

On June 17, 2015, representatives of GenSpera, Inc. (the “Company”) presented at the BIO International Convention 2015. The convention was held at the Pennsylvania Convention Center in Philadelphia, PA. Filed herewith as Exhibit 99.01 are the slides that were presented at the convention. The slides will also be available for at least 30 days in the “investors” section of the Company’s website at www.genspera.com.

The information contained in this Item 7.01 to this Current Report on Form 8-K and the exhibit attached hereto pertaining to this item shall not be deemed to be “filed” for 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, nor shall such information or such exhibits be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as shall be expressly set forth by specific reference in such a filing. The information set forth in the exhibits to this Form 8-K relating to this item 7.01 shall not be deemed an admission as to the materiality of any information in this report that is required to be disclosed solely to satisfy the requirements of Regulation FD.

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.

INDEX OF EXHIBITS