As filed with the Securities and Exchange Commission on March 19, 2019

Registration No. 333-                

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM S-8

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

MAGENTA THERAPEUTICS, INC.

(Exact name of registrant as specified in its charter)

100 Technology Square

Cambridge, Massachusetts 02139

(857) 242-0170

(Address of Principal Executive Offices)

MAGENTA THERAPEUTICS, INC. 2018 STOCK OPTION AND INCENTIVE PLAN

(Full title of the plan)

Jason Gardner

President and Chief Executive Officer

Magenta Therapeutics, Inc.

100 Technology Square

Cambridge, Massachusetts 02139

(Name and address of agent for service)

(857) 242-0170

(Telephone number, including area code, of agent for service)

Copies to:

Mitchell S. Bloom, Esq.

William D. Collins, Esq.

Goodwin Procter LLP

100 Northern Avenue

Boston, Massachusetts 02210

(617) 570-1000

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a 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.

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 pursuant to Section 7(a)(2)(B) of the Securities Act.  ☐

CALCULATION OF REGISTRATION FEE

EXPLANATORY NOTE

This Registration Statement on Form S-8 registers 1,332,201 additional shares of Common Stock under the Registrant’s 2018 Stock Option and Incentive Plan (the “Plan”). The number of shares of Common Stock reserved and available for issuance under the Plan is subject to an automatic annual increase on each January 1, beginning in 2019, by an amount equal to four percent of the number of shares of Common Stock issued and outstanding on the immediately preceding December 31 or such lesser number of shares of Common Stock as determined by the Administrator (as defined in the Plan). Accordingly, on January 1, 2019, the number of shares of Common Stock reserved and available for issuance under the Plan increased by 1,332,201. The additional shares are of the same class as other securities relating to the Plan for which the Registrant’s registration statement filed on Form S-8 (Registration No. 333-225838) on June 22, 2018, is effective. The information contained in the Registrant’s registration statement on Form S-8 (Registration No. 333-225838) is hereby incorporated by reference pursuant to General Instruction E.

Part II

INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 8. Exhibits.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Cambridge, Commonwealth of Massachusetts, on March 19, 2019.

POWER OF ATTORNEY AND SIGNATURES

KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below constitutes and appoints Jason Gardner and Jason Ryan, and each of them, as such person’s true and lawful attorneys-in-fact and agents, with full power of substitution and resubstitution in each of them, for such person in such person’s, place and stead, in any and all capacities, to sign for us and in our names in the capacities indicated below the Registration Statement on Form S-8 of Magenta Therapeutics, Inc., and any or all amendments (including post-effective amendments), and to file the same, with all exhibits thereto, and other documents in connection therewith, with the Securities and Exchange Commission, granting unto said attorneys-in-fact and agents, and each of them, full power and authority to do and perform each and every act and thing requisite or necessary to be done in connection therewith and about the premises, as fully to all intents and purposes as he might or could do in person, hereby ratifying and confirming all that said attorneys-in-fact and agents or any of them, or substitutes of any of them, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement on Form S-8 has been signed by the following persons in the capacities and on the dates indicated.

Exhibit 5.1

March 19, 2019

Magenta Therapeutics, Inc.

100 Technology Square

Cambridge, MA 02139

Re:    Securities Being Registered under Registration Statement on Form S-8

Ladies and Gentlemen:

We have acted as counsel to you in connection with your filing of a Registration Statement on Form S-8 (the “Registration Statement”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), on or about the date hereof relating to an aggregate of 1,332,201 shares (the “Shares”) of common stock, $0.001 par value per share (the “Common Stock”), of Magenta Therapeutics, Inc., a Delaware corporation (the “Company”), that may be issued pursuant to the Company’s 2018 Stock Option and Incentive Plan (the “Plan”).

We have reviewed such documents and made such examination of law as we have deemed appropriate to give the opinion set forth below. We have relied, without independent verification, on certificates of public officials and, as to matters of fact material to the opinion set forth below, on certificates of officers of the Company.

The opinion set forth below is limited to the Delaware General Corporation Law.

For purposes of the opinion set forth below, we have assumed that no event occurs that causes the number of authorized shares of Common Stock available for issuance by the Company to be less than the number of then unissued Shares.

Based on the foregoing, we are of the opinion that the Shares have been duly authorized and, upon issuance and delivery against payment therefor in accordance with the terms of the Plan, will be validly issued, fully paid and nonassessable.

We hereby consent to the inclusion of this opinion as Exhibit 5.1 to the Registration Statement. In giving our consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.

Very truly yours,

/s/ GOODWIN PROCTER LLP

GOODWIN PROCTER LLP

Exhibit 23.1

Consent of Independent Registered Public Accounting Firm

The Board of Directors

Magenta Therapeutics, Inc.:

We consent to the use of our report dated March 19, 2019 with respect to the consolidated balance sheets of Magenta Therapeutics, Inc. as of December 31, 2018 and 2017, the related consolidated statements of operations and comprehensive loss, redeemable convertible preferred stock and stockholders’ equity (deficit), and cash flows for each of the years in the three year period ended December 31, 2018, and the related notes (collectively, the “consolidated financial statements”), incorporated herein by reference.

/s/ KPMG LLP

Cambridge, Massachusetts

March 19, 2019