As filed with the Securities and Exchange Commission on June 17, 2014.

Registration No. 333-            

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-1

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

ZS PHARMA, INC.

(Exact name of Registrant as specified in its charter)

508 Wrangler Drive, Suite 100

Coppell, Texas 75019

(877) 700-0240

(Address, including zip code, and telephone number, including area code, of Registrant’s principal executive offices)

Todd A. Creech

Chief Financial Officer

ZS Pharma, Inc.

508 Wrangler Drive, Suite 100

Coppell, Texas 75019

(877) 700-0240

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Approximate date of commencement of proposed sale to the public:

As soon as practicable after the effective date of this Registration Statement.

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box.  ¨

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  x 333-195961

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

CALCULATION OF REGISTRATION FEE

The registration statement shall become effective upon filing with the Commission in accordance with Rule 462(b) under the Securities Act.

EXPLANATORY NOTE AND INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

Pursuant to Rule 462(b) under the Securities Act of 1933, as amended, ZS Pharma, Inc. (“Registrant”) is filing this registration statement with the Securities and Exchange Commission (“Commission”). This registration statement relates to the public offering of securities contemplated by the registration statement on Form S-1, as amended (File No. 333-195961) (“Prior Registration Statement”), and which the Commission declared effective on June 17, 2014.

The Registrant is filing this registration statement for the sole purpose of increasing the aggregate number of shares of common stock offered by the Registrant by 1,086,111 shares. The additional securities that are being registered for issuance and sale are in an amount and at a price that together represent no more than 20% of the maximum aggregate offering price set forth in the Calculation of Registration Fee table contained in the Prior Registration Statement. The information set forth in the Prior Registration Statement and all exhibits to the Prior Registration Statement are hereby incorporated by reference in this filing.

The required opinions and consents are listed on the Exhibit Index attached hereto and filed herewith.

Signatures

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in Coppell, Texas on June 17, 2014.

Pursuant to the requirements of the Securities Act, this Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

EXHIBIT INDEX