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): July 27, 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):

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company  ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.  ☐

Item 1.01 Entry Into a Material Definitive Agreement.

On July 27, 2017, Tetraphase Pharmaceuticals, Inc. (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with Piper Jaffray & Co. and BMO Capital Markets Corp., as representatives of the several underwriters (collectively, the “Underwriters”), relating to an underwritten public offering of 10,000,000 shares (the “Shares”) of the Company’s common stock, par value $0.001 per share (the “Common Stock”). All of the Shares are being sold by the Company. The offering price of the Shares to the public was $6.50 per share, and the Underwriters have agreed to purchase the Shares from the Company pursuant to the Underwriting Agreement at a price of $6.11 per share. Under the terms of the Underwriting Agreement, the Company granted the Underwriters an option, exercisable for 30 days, to purchase up to an additional 1,500,000 shares of Common Stock at the same price per share as the Shares. After underwriting discounts and commissions and estimated offering expenses, the Company expects to receive net proceeds from the offering of approximately $60.9 million (or approximately $70.1 million if the Underwriters exercise in full their option to purchase additional shares of common stock).

The Shares will be issued pursuant to a prospectus supplement dated July 27, 2017 and an accompanying base prospectus dated July 5, 2017 that form a part of the registration statement on Form S-3 that the Company filed with the Securities and Exchange Commission (“SEC”) on June 16, 2017 (File No. 333-218816). The closing of the offering is expected to take place on or about August 2, 2017, subject to the satisfaction of customary closing conditions.

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

A copy of the legal opinion and consent of Wilmer Cutler Pickering Hale and Dorr LLP relating to the Shares is attached as Exhibit 5.1 hereto.

Item 2.02 Results of Operations and Financial Condition.

On July 27, 2017, in a preliminary prospectus supplement filed with the SEC, the Company disclosed that its cash and cash equivalents as of June 30, 2017 was approximately $118.2 million.

The information contained in this Item 2.02 shall not be deemed “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 it be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as expressly set forth by specific reference in such a filing.

Item 8.01 Other Events.

The full text of the press release announcing the pricing of the underwritten public offering on July 27, 2017 is attached as Exhibit 99.1 hereto and is incorporated herein by reference.

Item 9.01. Financial Statements and Exhibits.

(d) Exhibits

The Exhibits to this Current Report on Form 8-K are listed in the Exhibit Index attached hereto.

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.

EXHIBIT INDEX