Sec Form 3 Filing - van der Horst Edward @ Sensei Biotherapeutics, Inc. - 2022-12-07

FORM 3 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person van der Horst Edward 2. Issuer Name and Ticker or Trading Symbol Sensei Biotherapeutics, Inc. [ SNSE] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) Chief Scientific Officer (Last) (First) (Middle) C/O SENSEI BIOTHERAPEUTICS, INC., 451 D STREET, SUITE 710 3. Date of Earliest Transaction (MM/DD/YY) 12/07/2022 (Street) BOSTON, MA02210 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 21,173( 1 ) D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Stock Option (Right to Buy) $ 16.32 ( 2 ) 08/28/2029 Common Stock 3,125 D Stock Option (Right to Buy) $ 3.22 ( 3 ) 08/04/2030 Common Stock 55,208 D Stock Option (Right to Buy) $ 19 ( 4 ) 01/26/2031 Common Stock 72,916 D Stock Option (Right to Buy) $ 8.69 ( 5 ) 10/30/2031 Common Stock 20,000 D Stock Option (Right to Buy) $ 4.3 ( 6 ) 02/14/2032 Common Stock 44,000 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other van der Horst Edward C/O SENSEI BIOTHERAPEUTICS, INC. 451 D STREET, SUITE 710 BOSTON, MA02210 Chief Scientific Officer Signatures /s/ Mark Ballantyne, Attorney-in-Fact 12/16/2022 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Represents: (1) 14,700 restricted stock units (the "RSUs"), with each RSU representing a contingent right to receive one share of the Issuer's common stock, vesting in four equal annual installments on each of February 15, 2023, February 15, 2024, February 15, 2025 and February 15, 2026, subject to the Reporting Person's continuous service with the Issuer through such vesting date; and (2) 6,473 shares purchased by the Reporting Person pursuant to the Issuer's 2021 Employee Stock Purchase Plan. ( 2 )25% of the shares subject to the option vest on the first anniversary of the vesting commencement date, September 3, 2021, and 1/48th of the shares subject to the option vest monthly thereafter over 36 months, subject to Reporting Person's continuous service as of each such date. ( 3 )25% of the shares subject to the option vest on the first anniversary of the vesting commencement date, February 15, 2022, and 1/48th of the shares subject to the option vest monthly thereafter over 36 months, subject to Reporting Person's continuous service as of each such date. ( 4 )25% of the shares subject to the option vest on the first anniversary of the vesting commencement date, February 4, 2023, and 1/48th of the shares subject to the option vest monthly thereafter over 36 months, subject to Reporting Person's continuous service as of each such date. ( 5 )25% of the shares subject to the option vest on the first anniversary of the vesting commencement date, October 8, 2023, and 1/48th of the shares subject to the option vest monthly thereafter over 36 months, subject to Reporting Person's continuous service as of each such date. ( 6 )25% of the shares subject to the option vest on the first anniversary of the vesting commencement date, February 15, 2024, and 1/48th of the shares subject to the option vest monthly thereafter over 36 months, subject to Reporting Person's continuous service as of each such date.

Remarks: Exhibit 24 - Power of Attorney

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