UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Commission File Number: 001-39980 Sensei Biotherapeutics, Inc. (Exact name of Registrant as specified in its Charter) Delaware 83-1863385 (State or other jurisdiction of incorporation or organization) 1405 Research Blvd, Suite 125 Rockville, MD 20850 (I.R.S. Employer Identification No.) FORM 10-Q (Mark One) ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended September ...

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Sensei Biotherapeutics, Inc. Washington, D.C. 20549 (Exact name of Registrant as specified in its Charter) FORM 10-Q (Mark One) ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended June 30, 2021 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 FOR THE TRANSITION PERIOD FROM TO Commission File Number: 001-39980 | Delaware | 83-1863385 | (State or ot ...

Washington, D.C. 20549 UNITED STATES SECURITIES AND EXCHANGE COMMISSION FORM 10-Q (Mark One) ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended March 31, 2021 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 FOR THE TRANSITION PERIOD FROM TO Commission File Number: 001-39980 Sensei Biotherapeutics, Inc. (Exact name of Registrant as specified in its Charter) Delaware 83-1863385 (State or other j ...

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 TO Commission File Number: 001-39980 PART I Item 1. Business. Overview Sensei Biotherapeutics, Inc. (Exact name of Registrant as specified in its Charter) Delaware ...