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 7, 2016

ARDELYX, INC.

(Exact name of registrant as specified in its charter)

34175 Ardenwood Blvd., Suite 200

Fremont, CA 94555

(Address of principal executive offices, including Zip Code)

Registrant’s telephone number, including area code: (510) 745-1700

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:

On January 7, 2016, Ardelyx, Inc. (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with Citigroup Global Markets Inc. and Leerink Partners LLC, as representatives (the “Representatives”) of the several underwriters named therein (collectively, the “Underwriters”), pursuant to which the Company agreed to issue and sell 7,500,000 shares (the “Shares”) of its common stock, par value $0.0001 per share (“Common Stock”), to the Underwriters (the “Offering”). The Shares were sold at a public offering price of $10.00 per Share, and were purchased by the Underwriters from the Company at a price of $9.40 per Share. Under the terms of the Underwriting Agreement, the Company granted the Underwriters the right, for 30 days, to purchase up to 1,125,000 additional shares of Common Stock at the public offering price. On January 8, 2016, the Underwriters exercised their right to purchase the additional 1,125,000 shares of Common Stock in full.

The Offering was made under a prospectus supplement and related prospectus filed with the Securities and Exchange Commission pursuant to the Company’s effective shelf registration statement on Form S-3 (Registration No. 333-205631).

On January 13, 2016, the Offering closed and the Company completed the sale and issuance of an aggregate of 8,625,000 shares of Common Stock. The Company received net proceeds from the Offering of approximately $80.4 million, after deducting the Underwriters’ discounts and commissions and estimated offering expenses payable by the Company. As of December 31, 2015, the Company had cash and cash equivalents of approximately $105.0 million and, after giving effect to the issuance and sale of shares of Common Stock in the Offering, approximately $185.4 million in cash and cash equivalents.

Pursuant to the Underwriting Agreement, the Company agreed to indemnify the Underwriters against certain liabilities, including liabilities under the Securities Act of 1933, as amended, or to contribute to payments that the Underwriters may be required to make because of such liabilities. The Company and all of the Company’s directors and executive officers also agreed not to sell or transfer any Common Stock held by them for 60 days after January 7, 2016 without first obtaining the written consent of the Representatives on behalf of the Underwriters, subject to certain exceptions as described in the prospectus supplement.

A copy of the Underwriting Agreement is attached as Exhibit 1.1 hereto and is incorporated herein by reference. The foregoing descriptions of the Underwriting Agreement and lock-up arrangements do not purport to be complete and are qualified in their entirety by reference to such exhibit.

A copy of the opinion of Latham & Watkins LLP relating to the validity of the securities issued in the Offering is filed herewith as Exhibit 5.1.

(d) Exhibits.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.