UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No. ___________)*

(Name of Issuer)

(Title of Class of Securities)

(CUSIP Number)

(Date of Event Which Requires Filing of This Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

*The remainder of this cover page shall be filled out for a reporting person's initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

(1) As described in Item 5 below, Atlas Venture Fund X, L.P., a Delaware limited partnership (“Atlas X”), Atlas Venture Associates X, L.P., a Delaware limited partnership (“AVA X LP”) and Atlas Venture Associates X, LLC, a Delaware limited liability company (“AVA X LLC” and together with Atlas X and AVA X LP, the “Reporting Persons”) beneficially own 4,453,386 shares of the Issuer’s Common Stock. All of these shares are directly held by Atlas X. AVA X LP is the general partner of Atlas X and AVA X LLC is the general partner of AVA X LP. Each of AVA X LP and AVA X LLC has voting and dispositive power over the shares held by Atlas X. As such, each of the Reporting Persons share voting and dispositive power with respect to the shares held by Atlas X.

(2) This percentage is calculated based upon 31,553,672 outstanding shares of Common Stock of the Issuer as of November 12, 2018, as reported in the Issuer’s quarterly report on Form 10-Q for the quarter ended September 30, 2018 and filed with the Securities and Exchange Commission on November 14, 2018.

(1) As described in Item 5 below, Atlas Venture Fund X, L.P., a Delaware limited partnership (“Atlas X”), Atlas Venture Associates X, L.P., a Delaware limited partnership (“AVA X LP”) and Atlas Venture Associates X, LLC, a Delaware limited liability company (“AVA X LLC” and together with Atlas X and AVA X LP, the “Reporting Persons”) beneficially own 4,453,386 shares of the Issuer’s Common Stock. All of these shares are directly held by Atlas X. AVA X LP is the general partner of Atlas X and AVA X LLC is the general partner of AVA X LP. Each of AVA X LP and AVA X LLC has voting and dispositive power over the shares held by Atlas X. As such, each of the Reporting Persons share voting and dispositive power with respect to the shares held by Atlas X.

(2) This percentage is calculated based upon 31,553,672 outstanding shares of Common Stock of the Issuer as of November 12, 2018, as reported in the Issuer’s quarterly report on Form 10-Q for the quarter ended September 30, 2018 and filed with the Securities and Exchange Commission on November 14, 2018.

(1) As described in Item 5 below, Atlas Venture Fund X, L.P., a Delaware limited partnership (“Atlas X”), Atlas Venture Associates X, L.P., a Delaware limited partnership (“AVA X LP”) and Atlas Venture Associates X, LLC, a Delaware limited liability company (“AVA X LLC” and together with Atlas X and AVA X LP, the “Reporting Persons”) beneficially own 4,453,386 shares of the Issuer’s Common Stock. All of these shares are directly held by Atlas X. AVA X LP is the general partner of Atlas X and AVA X LLC is the general partner of AVA X LP. Each of AVA X LP and AVA X LLC has voting and dispositive power over the shares held by Atlas X. As such, each of the Reporting Persons share voting and dispositive power with respect to the shares held by Atlas X.

(2) This percentage is calculated based upon 31,553,672 outstanding shares of Common Stock of the Issuer as of November 12, 2018, as reported in the Issuer’s quarterly report on Form 10-Q for the quarter ended September 30, 2018 and filed with the Securities and Exchange Commission on November 14, 2018.

Replimune Group, Inc. (the “Issuer”)

18 Commerce Way

Woburn, MA 01801

This Schedule 13G is filed by (i) Atlas Venture Fund X, L.P., a Delaware limited partnership (“Atlas X”), (ii) Atlas Venture Associates X, L.P., a Delaware limited partnership (“AVA X LP”) and (iii) Atlas Venture Associates X, LLC, a Delaware limited liability company (“AVA X LLC” and together with Atlas X and AVA X LP, the “Reporting Persons”).

400 Technology Square, 10th Floor

Cambridge, Massachusetts 02139.

Each of Atlas X and AVA X LP is a Delaware limited partnership. AVA X LLC is a Delaware limited liability company.

Common Stock, $0.001 par value

76029N106

Not applicable.

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1

(a) Amount beneficially owned: Atlas X is the record owner of 4,453,386 shares of Common Stock. AVA X LP is the general partner of Atlas X and AVA X LLC is the general partner of AVA X LP. Each of Atlas X, AVA X LP and AVA X LLC has shared voting and dispositive power over the shares held by Atlas X. As such, each of Atlas X, AVA X LP and AVA X LLC may be deemed to beneficially own the shares held by Atlas X.

(b) Percent of class: Each of the Reporting Persons may be deemed to beneficially own 14.1% of the Issuer’s outstanding Common Stock, which percentage is calculated based upon 31,553,672 outstanding shares of Common Stock of the Issuer as of November 12, 2018, as reported in the Issuer’s quarterly report on Form 10-Q for the quarter ended September 30, 2018 and filed with the Securities and Exchange Commission on November 14, 2018.

(c) Number of shares as to which the person has:

(i) Sole power to vote or to direct the vote: None of the Reporting Persons have the sole power to direct the vote of the Common Stock.

(ii) Shared power to vote or to direct the vote: Each Reporting Person shares power to vote or direct the vote of 4,453,386 shares of Common Stock.

(iii) Sole power to dispose or to direct the disposition of: None of the Reporting Persons have the sole power to dispose or to direct the disposition of the Common Stock.

(iv) Shared power to dispose or to direct the disposition of: Each Reporting Person shares power to dispose or to direct the disposition of 4,453,386 shares of Common Stock.

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following. ¨

Not applicable.

Not applicable.

Not applicable.

Not applicable.

Not applicable.

CUSIP No. 76029N106

Page 7 of 8 Pages

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: February 13, 2019.

EXHIBITS

Exhibit A

JOINT FILING AGREEMENT

In accordance with Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended, the undersigned agree to the joint filing on behalf of each of them of a statement on Schedule 13G (including amendments thereto) with respect to the Common Stock of Replimune Group, Inc. and further agree that this agreement be included as an exhibit to such filing. Each party to the agreement expressly authorizes each other party to file on its behalf any and all amendments to such statement. Each party to this agreement agrees that this joint filing agreement may be signed in counterparts.

IN WITNESS WHEREOF, the undersigned have executed this Joint Filing Agreement as of February 13, 2019.