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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - K ( Mark One ) 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 ΤΟ Commission File Number 001-38663 Gritstone Oncology , Inc. ( Exact name of Registrant as specified in its Charter ) Delaware ( State or other jurisdiction of incorporation or organization ) 5959 Horton Street , Suite 300 Emeryville , CA ( Address of principal executive offices ) ( 510 ) 871-6100 Registrant's telephone number , including area code Securities registered pursuant to Section 12 ( b ) of the Act : Trading Symbol ( s ) GRTS Title of each class Common Stock , $ 0.0001 par value per share 47-4859534 ( I.R.S. Employer Identification No. ) 94608 ( Zip Code ) Name of each exchange on which registered The Nasdaq Global Select Market 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 . YESNO > 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 , smaller reporting company , or an emerging growth company . See the 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 Non - accelerated filer Accelerated filer Smaller reporting company 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 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 ark ther the Registrant is a shell company ( as defined in Rule 12b - 2 of the Exchange Act ) . YES NO The aggregate market value of the common stock held by non - affiliates of the registrant as of June 30 , 2020 ( the last business day of the registrant's most recently completed second fiscal quarter ) was approximately $ 198.7 million , based on the closing price of the registrant's common stock , as reported by the NASDAQ Global Select Market on June 30 , 2020 of $ 6.64 per share . Shares of the registrant's common stock held by each executive officer , director , and holder of 5 % or more of the outstanding common stock have been excluded in that such persons may deemed to be affiliates . This calculation does not reflect a determination that certain persons are affiliates of the registrant for any other purpose . The number of shares of Registrant's Common Stock outstanding as of March 5 , 2021 was 48,951,561 . DOCUMENTS INCORPORATED BY REFERENCE Portions of the Registrant's Definitive Proxy Statement relating to the Annual Meeting of Shareholders , scheduled to be held on June 18 , 2021 , are incorporated by reference into Part III of this Annual Report on Form 10 - K where indicated . Such Definitive Proxy Statement will be filed with the Securities and Exchange Commission within 120 days after the fiscal year to which this report relates .