Atara Biotherapeutics, Inc.

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 all boxes that apply):

1280 Rancho Conejo Boulevard

Thousand Oaks, CA 91320

NOTICE OF ANNUAL MEETING OF STOCKHOLDERS

To Be Held on June 9, 2026

Dear Stockholder:

You are cordially invited to attend the Annual Meeting of Stockholders (the “Annual Meeting”) of Atara Biotherapeutics, Inc., a Delaware corporation. The meeting will be held virtually via live audio webcast on June 9, 2026 at 9:00 a.m. Pacific time for the following purposes:

To elect our two nominees for director named in the accompanying Proxy Statement to hold office until the 2029 Annual Meeting of Stockholders;

To approve, on an advisory basis, the compensation of our named executive officers, as disclosed in the Proxy Statement accompanying this Notice;

To approve the first amendment to our 2024 Equity Incentive Plan to increase the number of shares of common stock reserved for issuance thereunder by 400,000;

To ratify the selection by the Audit Committee of our Board of Directors of Deloitte & Touche LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2026; and

To conduct any other business properly brought before the Annual Meeting.

These items of business are more fully described in the Proxy Statement accompanying this Notice. This Notice is not a form for voting and presents only an overview of the more complete proxy materials, which contain important information. You should access and review all proxy materials before voting. The Proxy Statement accompanying this Notice contains instructions on how to access our proxy materials and vote online, or in the alternative, request a paper copy of the proxy materials and a proxy card. The record date for the Annual Meeting is April 14, 2026 (the “Record Date”). Only stockholders of record at the close of business on that date may vote at the meeting or any adjournment thereof.

To increase access for all our stockholders, we have determined that it is prudent to hold this year’s Annual Meeting in a virtual-only format via live audio webcast. You may attend the virtual Annual Meeting at www.virtualshareholdermeeting.com/ATRA2026. To participate in the Annual Meeting, you will need the 16-digit control number that appears on your Notice of Internet Availability of Proxy Materials, proxy card or the instructions that accompanied your proxy materials. Refer to the “Questions and Answers About These Proxy Materials and Voting” section of the accompanying Proxy Statement for detailed procedures regarding attending, submitting questions and voting at the virtual Annual Meeting.

A list of stockholders entitled to vote at the meeting will be available for examination during normal business hours for ten days prior to the meeting for any purpose germane to the meeting at our corporate headquarters at 1280 Rancho Conejo Boulevard, Thousand Oaks, CA 91320.

By Order of the Board of Directors

/s/AnhCo Nguyen

AnhCo Nguyen, Ph.D.

President and Chief Executive Officer

Thousand Oaks, California

April 24, 2026

Important Notice Regarding the Availability of Proxy Materials for the Annual Meeting of Stockholders to Be Held on June 9, 2026 at 9:00 a.m. Pacific time held virtually at www.virtualshareholdermeeting.com/ATRA2026. The Proxy Statement and Atara’s Annual Report on Form 10-K for the fiscal year 2025 are available electronically at www.proxyvote.com.

Proxy materials or this Notice are being first released or mailed on or about April 27, 2026, to all stockholders entitled to vote at the Annual Meeting. In accordance with rules and regulations adopted by the Securities and Exchange Commission, instead of mailing a printed copy of our proxy materials to each record stockholder, we may furnish proxy materials by providing internet access to those documents.

You are cordially invited to attend the Annual Meeting virtually via live audio webcast. Whether or not you expect to attend the meeting, please complete, date, sign and return the proxy card mailed to you, or vote over the telephone or the internet as instructed in these materials, as promptly as possible in order to ensure your representation at the meeting. Stockholders who attend the virtual Annual Meeting should follow instructions at www.virtualshareholdermeeting.com/ATRA2026 to vote online during the Annual Meeting. Please note, however, that if your shares are held of record by a broker, bank or other nominee and you wish to vote at the virtual Annual Meeting, you must follow the instructions from that record holder.

TABLE OF CONTENTS

1280 Rancho Conejo Boulevard, Thousand Oaks, CA 91320

PROXY STATEMENT

FOR THE 2026 ANNUAL MEETING OF STOCKHOLDERS