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( Mark One ) UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - K ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31 , 2020 OR TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from ATARA BIOTHERAPEUTICS , INC . ( Exact name of Registrant as specified in its Charter ) Delaware ( State or other jurisdiction of incorporation or organization ) 611 Gateway Blvd. , Suite 900 South San Francisco , CA ( Address of principal executive offices ) Commission File Number 001-36548 Securities registered pursuant to Section 12 ( b ) of the Act : to Title of each class Common Stock , par value $ 0.0001 per share , Registrant's telephone number , including area code : ( 650 ) 278-8930 Trading Symbol ( s ) ATRA 46-0920988 ( I.R.S. Employer Identification No. ) 94080 ( Zip Code ) Name of each exchange on which registered The Nasdaq Stock Market LLC Securities registered pursuant to Section 12 ( g ) of the Act : None Indicate by check mark if the Registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . YES NO Indicate by check mark if the Registrant is not required to file reports pursuant to Section 13 or 15 ( d ) of the Act . YES NO X Indicate by check mark whether the Registrant : ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the Registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . YES NO Indicate by check mark whether the Registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( §232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the Registrant was required to submit such files ) . YES NO Indicate by check mark whether the Registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company , or an emerging growth company . See definitions of " large accelerated filer " , " accelerated filer ” , “ smaller reporting company " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Large accelerated filer Accelerated filer Non - accelerated filer Small reporting company The number of outstanding shares of the Registrant's Common Stock as of February 18 , 2021 was 83,614,853 . DOCUMENTS INCORPORATED BY REFERENCE ☐ Emerging growth company ■ If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . Indicate by check mark whether the registrant has filed a report on and attestation to its management’s assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . Indicate by check mark whether the Registrant is a shell company ( as defined in Rule 12b - 2 of the Exchange Act ) . YES NO > The aggregate market value of common stock held by non - affiliates of the Registrant , based on the closing sales price for such stock on June 30 , 2020 as reported by The Nasdaq Stock Market , was $ 906,895,284 . This calculation excludes 12,063,880 shares held by executive officers , directors and stockholders that the Registrant has concluded are affiliates of the Registrant . Exclusion of such shares should not be construed to indicate that any such person possesses the power , direct or indirect , to direct or cause the direction of the management or policies of the registrant or that such person is controlled by or under common control with the Registrant . Portions of the Registrant's definitive proxy statement relating to its 2021 Annual Meeting of Stockholders are incorporated by reference into Part III of this Report where indicated . Such proxy statement will be filed with the U.S. Securities and Exchange Commission within 120 days after the end of the fiscal year to which this report relates .