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): August 18, 2016

Eiger BioPharmaceuticals, Inc.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (650) 272-6138

(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 obligations of the registrant under any of the following provisions:

On August 18, 2016, Eiger BioPharmaceuticals, Inc. (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with Jefferies LLC, as representative of the several underwriters (the “Underwriters”) relating to the offering, issuance and sale (the “Offering”) of 1,250,000 shares of the Company’s common stock, par value $0.001 per share. The price to the public in the Offering is $16.00 per share. Pursuant to the Underwriting Agreement, the Underwriters have agreed to purchase shares of common stock from the Company at a price of $15.04 per share, which will result in approximately $18.8 million of net proceeds to the Company after deducting the underwriting discount and estimated offering expenses. The Offering is expected to close on or about August 23, 2016, subject to customary closing conditions. The shares of common stock will be listed on The NASDAQ Global Market. The Underwriters have a 30-day option to purchase up to an additional 187,500 shares of common stock. All of the shares in the Offering are being sold by the Company. Pursuant to the terms of the Underwriting Agreement, each of the Company’s directors and officers have entered into “lock-up” agreements with Underwriter that generally prohibit, without prior written consent of Jefferies LLC, the sale, transfer or other disposition of securities of the Company for a period ending 90 days after August 18, 2016.

The Offering is being made pursuant to the Company’s effective shelf registration statement on Form S-3 (Registration No. 333-212114) filed with the Securities and Exchange Commission (the “SEC”) on June 17, 2016 and declared effective on August 4, 2016. The Company has filed a final prospectus supplement, dated August 18, 2016, relating to the issuance and sale of the shares with the SEC.

The Underwriting Agreement contains customary representations, warranties and agreements by the Company, customary conditions to closing, indemnification obligations of the Company and the Underwriters, including for liabilities under the Securities Act of 1933, as amended, other obligations of the parties and termination provisions. The representations, warranties and covenants contained in the Underwriting Agreement were made only for purposes of such agreement and as of specific dates, were solely for the benefit of the parties to such agreement and may be subject to limitations agreed upon by the contracting parties.

The Underwriting Agreement is filed as Exhibit 1.1 hereto and is incorporated herein by reference. The foregoing description of the terms of the Underwriting Agreement is qualified in its entirety by reference to such exhibit. A copy of the opinion of Cooley LLP relating to the legality of the issuance and sale of the shares in the Offering is attached as Exhibit 5.1 hereto.

Forward-Looking Statements

The statements in this report related to the completion, timing and size of the Offering are “forward-looking” statements. These forward-looking statements are based upon the Company’s current expectations. Forward-looking statements involve risks and uncertainties. The Company’s actual results and the timing of events could differ materially from those anticipated in such forward-looking statements as a result of these risks and uncertainties, which include, without limitation, risks related to market conditions and the satisfaction of customary closing conditions related to the Offering. There can be no assurance that the Company will be able to complete the Offering on the anticipated terms, or at all.

(d) Exhibits.

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