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 9, 2015

Calithera Biosciences, Inc.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (650) 870-1000

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:

Item 5.07. Submission of Matters to a Vote of Security Holders.

On June 9, 2015, Calithera Biosciences, Inc. (“Calithera”) held its 2015 Annual Meeting of Stockholders (the “Annual Meeting”) at Embassy Suites Hotel, 250 Gateway Boulevard, South San Francisco, California 94080. At the Annual Meeting, Calithera’s stockholders voted on two proposals, each of which is described in more detail in Calithera’s definitive proxy statement on Schedule 14A filed with the U.S. Securities and Exchange Commission on April 24, 2015. The following is a brief description of each matter voted upon and the results, including the number of votes cast for and against each matter and, if applicable, the number of abstentions and broker non-votes with respect to each matter.

Proposal 1. Stockholders elected the one nominee for Class I director to serve until Calithera’s 2018 Annual Meeting of Stockholders or until her respective successor has been duly elected and qualified. The voting results were as follows:

Proposal 2. Stockholders ratified the selection by the Audit Committee of the Board of Directors of Calithera of Ernst & Young LLP as Calithera’s independent registered public accounting firm for the year ending December 31, 2015. The voting results were as follows:

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.