As filed with the Securities and Exchange Commission on July 9, 2015

Registration No. 333-            

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-1

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

ATARA BIOTHERAPEUTICS, INC.

(Exact Name of Registrant as Specified in Its Charter)

701 Gateway Boulevard, Suite 200

South San Francisco, California 94080

(650) 278-8930

(Address, Including Zip Code and Telephone Number, of Registrant’s Principal Executive Offices)

Isaac E. Ciechanover, M.D.

Chief Executive Officer

Atara Biotherapeutics, Inc.

701 Gateway Boulevard, Suite 200

South San Francisco, California 94080

(650) 278-8930

(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 this Registration Statement becomes effective.

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, 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    (File No. 333-205347)

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

This Registration Statement shall become effective upon filing 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, Atara Biotherapeutics, Inc. (the “Registrant”) is filing this Registration Statement on Form S-1 (this “Registration Statement”) with the Securities and Exchange Commission (the “Commission”) solely for the purpose of registering an increase in the maximum aggregate offering price by $34,500,000. This Registration Statement relates to the public offering of shares of common stock contemplated by the Registration Statement on Form S-1, as amended (File No. 333-205347) (the “Prior Registration Statement”), which the Registrant originally filed with the Commission on June 29, 2015, and which the Commission declared effective on July 9, 2015. Pursuant to Rule 462(b), the information set forth in the Prior Registration Statement and all exhibits to the Prior Registration Statement are hereby incorporated by reference into this Registration Statement.

The required opinion 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 on Form S-1 to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of South San Francisco, State of California, on this 9^th day of July, 2015.

Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

EXHIBIT INDEX