Atara Biotherapeutics (ATRA) S-8 Registration of securities for employees

Exhibit 5.1

SIDLEY AUSTIN LLP ONE SOUTH DEARBORN STREET CHICAGO, IL 60603 +1 312 853 7000 +1 312 853 7036 FAX   AMERICA • ASIA PACIFIC • EUROPE

July 22, 2022

Atara Biotherapeutics, Inc.

611 Gateway Blvd., Suite 900

South San Francisco, California 94080

Re: 1,500,000 Shares of Common Stock, $0.0001 par value per share (“Common Stock”)

Ladies and Gentlemen:

We refer to the Registration Statement on Form S-8 (the “Registration Statement”) being filed by Atara Biotherapeutics, Inc., a Delaware corporation (the “Company”), with the Securities and Exchange Commission under the Securities Act of 1933, as amended (the “Securities Act”), relating to the registration of 1,500,000 shares of Common Stock (the “Registered Shares”) of the Company which may be issued under the Atara Biotherapeutics, Inc. Third Amended and Restated 2018 Inducement Plan (the “Plan”).

This opinion letter is being delivered in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act.

We have examined the Registration Statement, the Company’s Amended and Restated Certificate of Incorporation, the Company’s Amended and Restated Bylaws, the Plan, and the resolutions adopted by the board of directors of the Company relating to the Registration Statement and the Plan. We have also examined originals, or copies of originals certified to our satisfaction, of such agreements, documents, certificates and statements of the Company and other corporate documents and instruments, and have examined such questions of law, as we have considered relevant and necessary as a basis for this opinion letter. We have assumed the authenticity of all documents submitted to us as originals, the genuineness of all signatures, the legal capacity of all persons and the conformity with the original documents of any copies thereof submitted to us for examination. As to facts relevant to the opinions expressed herein, we have relied without independent investigation or verification upon, and assumed the accuracy and completeness of, certificates, letters and oral and written statements and representations of public officials and officers and other representatives of the Company.

Based on the foregoing, we are of the opinion that each Registered Share that is newly issued pursuant to the Plan will be validly issued, fully paid and non-assessable when: (i) the Registration Statement, as finally amended, shall have become effective under the Securities Act; (ii) such Registered Share shall have been duly issued and delivered in accordance with the Plan; and (iii) a certificate representing such Registered Share shall have been duly executed,

Sidley Austin LLP is a limited liability partnership practicing in affiliation with other Sidley Austin partnerships.

Atara Biotherapeutics, Inc.

July 22, 2022

Page 2

countersigned and registered and duly delivered to the person entitled thereto against payment of the agreed consideration therefor (in an amount not less than the par value thereof) or, if any Registered Share is to be issued in uncertificated form, the Company’s books shall reflect the issuance of such Registered Share to the person entitled thereto against payment of the agreed consideration therefor (in an amount not less than the par value thereof) all in accordance with the Plan.

This opinion letter is limited to the General Corporation Law of the State of Delaware. We express no opinion as to the laws, rules or regulations of any other jurisdiction, including, without limitation, the federal laws of the United States of America or any state securities or blue sky laws.

We hereby consent to the filing of this opinion letter as an exhibit to the Registration Statement and to all references to our Firm included in or made a part of the Registration Statement. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act.

Very truly yours,

/s/ Sidley Austin LLP