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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - K ( Mark One ) X 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 R PORT PURSU ANT TO SECTION 13 OR 15 ( d ) OF THE THE TRANSITION PERIOD FROM ΤΟ Delaware ( State or other jurisdiction of incorporation or organization ) 1405 Research Blvd , Suite 125 Sensei Biotherapeutics , Inc. ( Exact name of Registrant as specified in its Charter ) Rockville , MD ( Address of principal executive offices ) Commission File Number : 001-39980 Securities registered pursuant to Section 12 ( b ) of the Act : Title of each class Common Stock , $ 0.0001 par value per share RITIES EXCHANGE ACT OF 1934 FOR Trading Symbol ( s ) SNSE 83-1863385 ( I.R.S. Employer Identification No. ) Registrant's telephone number , including area code : ( 240 ) 243-8000 20850 ( Zip Code ) Name of each exchange on which registered The Nasdaq Stock Market LLC Securities registered pursuant to Section 12 ( g ) 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 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 L Non - accelerated filer Accelerated filer Smaller reporting company Emerging growth company I 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 There was no aggregate market value of shares of 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 , because the registrant's common stock was not trading on any exchange on that date . The number of shares of Registrant's Common Stock outstanding as of March 25 , 2021 was 30,588,495 .