UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a)

of the Securities Exchange Act of 1934

(Amendment No.__)

Check the appropriate box:

Atara Biotherapeutics, Inc.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement if Other Than the Registrant)

Payment of Filing Fee (Check the appropriate box):

Your Vote Counts! ATARA BIOTHERAPEUTICS, INC. 2024 Annual Meeting Vote by June 9, 2024 11:59 PM ET ATARA BIOTHERAPEUTICS, INC. 2380 CONEJO SPECTRUM STREET, SUITE 200 THOUSAND OAKS, CA 91320 V46744-P08093 You invested in ATARA BIOTHERAPEUTICS, INC. and it’s time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy materials for the stockholder meeting to be held on June 10, 2024. Get informed before you vote View the Notice and Proxy Statement and 2023 Annual Report on Form 10-K online OR you can receive a free paper or email copy of the material(s) by requesting prior to May 27, 2024. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. For complete information and to vote, visit www.ProxyVote.com Control # Smartphone users Point your camera here and vote without entering a control number Vote Virtually at the Meeting* June 10, 2024 9:00 AM Pacific Time Virtually at: www.virtualshareholdermeeting.com/ATRA2024 *Please check the meeting materials for any special requirements for meeting attendance.

Vote at www.ProxyVote.com THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming stockholder meeting. Please follow the instructions on the reverse side to vote these important matters. Board Recommends Voting Items 1. Election of Directors Nominees: 1a. William K. Heiden For 1b. Ameet Mallik For 2. To approve, on an advisory basis, the compensation of the Company’s named executive officers, as disclosed in the Proxy Statement. For 3. To ratify the selection of Deloitte & Touche LLP as the Company’s independent registered public accounting firm for the fiscal year ending December 31, 2024. For 4. To approve the Company’s 2024 Equity Incentive Plan. For 5. To approve an automatic annual increase to the Company’s 2024 Equity Incentive Plan. For 6. To approve an increase in the number of shares of common stock available for issuance under the Company’s 2014 Employee Stock Purchase Plan. For 7. To approve an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse stock split of the Company’s common stock at a ratio ranging from any whole number between 1-for-4 and1-for-30, as determined by the Board of Directors in its discretion, subject to the Board of Directors’ authority to abandon such amendments (the “Charter Amendment Proposal”). For 8. To approve the adjournment of the Annual Meeting to a later date or dates, if necessary, to permit further solicitation and vote of proxies in the event there are not sufficient votes in favor of the Charter Amendment Proposal. For NOTE: Such other matters that may properly come before the meeting or any adjournment or postponement thereof will be voted on by the proxy holders in their discretion. Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click “Delivery Settings”. V46745-P08093