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): January 17, 2017

Tetraphase Pharmaceuticals, Inc.

(Exact name of registrant as specified in charter)

Registrant’s telephone number, including area code: (617) 715-3600

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 Instruction A.2. below):

On January 17, 2017, Tetraphase Pharmaceuticals, Inc. (the “Company”) entered into a Controlled Equity Offering^SM Sales Agreement, dated January 17, 2017 (the “Sales Agreement”) with Cantor Fitzgerald & Co., as agent (“Cantor Fitzgerald”), pursuant to which the Company may issue and sell shares of its common stock, $0.001 par value per share, having an aggregate offering price of up to $40,000,000 (the “Shares”) from time to time through Cantor Fitzgerald (the “Offering”). Also on January 17, 2017, the Company filed a prospectus supplement with the Securities and Exchange Commission in connection with the Offering (the “Prospectus Supplement”) under its existing Registration Statement on Form S-3 (File No 333-214500), which became effective on December 7, 2016 (the “Registration Statement”).

Upon delivery of a placement notice by the Company and subject to the terms and conditions of the Sales Agreement, Cantor Fitzgerald may sell the Shares in accordance with the terms set forth in the placement notice and by methods deemed to be an “at the market offering” as defined in Rule 415(a)(4) promulgated under the Securities Act of 1933, as amended (the “Securities Act”), including sales made directly on or through The NASDAQ Global Select Market or on any other existing trading market for the Company’s common stock. Subject to the terms of a placement notice, Cantor Fitzgerald may also sell the Shares in negotiated transactions at market prices prevailing at the time of sale and/or any other method permitted by law, subject to the Company’s prior written consent.

The Company or Cantor Fitzgerald may suspend or terminate the offering of Shares upon notice to the other party, subject to certain conditions. Under the Sales Agreement, Cantor Fitzgerald has agreed to use commercially reasonable efforts consistent with its normal trading and sales practices and applicable state and federal law, rules and regulations and the rules of NASDAQ to sell the Shares in accordance with the terms of the applicable placement notice.

The Company has agreed to pay Cantor Fitzgerald commissions for its services in acting as agent in the sale of the Shares in the amount equal to 3.0% of gross proceeds from the sale of the Shares pursuant to the Sales Agreement. The Company has also agreed to provide Cantor Fitzgerald with certain customary indemnification and contribution rights.

A copy of the Sales Agreement is attached as Exhibit 1.1 hereto and is incorporated herein by reference. The foregoing description of the material terms of the Sales Agreement does not purport to be complete and is qualified in its entirety by reference to such exhibit.

Wilmer Cutler Pickering Hale and Dorr LLP, counsel to the Company, has issued a legal opinion relating to the Shares. A copy of such legal opinion, including the consent included therein, is attached as Exhibit 5.1 hereto.

The Shares will be sold pursuant to the Registration Statement, and offerings of the Shares will be made only by means of the Prospectus Supplement. This Current Report on Form 8-K shall not constitute an offer to sell or solicitation of an offer to buy these securities, nor shall there be any sale of these securities in any state in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities law of such state or jurisdiction.

See Exhibit Index attached hereto.

SIGNATURE

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.

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