Exhibit 5.1

767 Fifth Avenue

New York, NY 10153-0119

+1 212 310 8000 tel

+1 212 310 8007 fax

April 11, 2022

ATI Physical Therapy, Inc.

790 Remington Boulevard

Bolingbrook, Illinois 6044

Ladies and Gentlemen:

We have acted as counsel to ATI Physical Therapy, Inc., a Delaware corporation (the “Company”), in connection with the preparation and filing with the Securities and Exchange Commission of the Company’s Registration Statement on Form S-1 (the “Registration Statement”), pursuant to the Securities Act of 1933, as amended (the “Act”), relating to the offer and sale by the selling securityholders (the “Selling Securityholders”) named in the prospectus contained in the Registration Statement of:

(a) shares of common stock, par value $0.0001 per share (“Common Stock”) of the Company (the “Selling Securityholder Shares”);

(b)(i) warrants sold pursuant to the Series A Senior Preferred Stock Purchase Agreement, dated as of February 24, 2022, by and among the purchasers signatory thereto (the “Preferred Stock Purchase Agreement”), each exercisable for one share of Common Stock at an exercise price of $3.00 per share Series I Warrants (the “Series I Warrants”) and (ii) warrants sold pursuant to the Preferred Stock Purchase Agreement, each exercisable for one share of Common Stock at an exercise price of $0.01 per share (the “Series II Warrants” and, together with the Series I Warrants, the “Selling Securityholder Warrants”); and

(c)(i) shares of Common Stock issuable upon the exercise of the Series I Warrants and (ii) shares of Common Stock issuable upon the exercise of the Series II Warrants (collectively, the “Warrant Shares”).

For purposes of this opinion letter, the following terms have the meanings specified below:

In so acting, we have examined originals or copies (certified or otherwise identified to our satisfaction) of (i) the Second Amended and Restated Certificate of Incorporation of the Company filed with the Secretary of State of the State of Delaware incorporated by reference as Exhibit 3.1 to the Registration Statement; (ii) the Amended and Restated Bylaws of the Company, incorporated by reference as Exhibit 3.2 to the Registration Statement; (iii) the Registration Statement; (iv) the prospectus contained within the Registration Statement; (v) the Warrant Agreement, dated as of February 24, 2022, by and between the Company and Continental Stock Transfer & Trust Company (the “Warrant Agreement”), incorporated by reference as Exhibit 4.4 to the Registration Statement; (vi) the Preferred Stock Purchase Agreement, incorporated by reference as Exhibit 10.25 to the Registration Statement; and (vi) such corporate records, agreements, documents and other instruments, and such certificates or comparable documents of public officials and of officers and representatives of the Company, and have made such inquiries of such officers and representatives, as we have deemed relevant and necessary as a basis for the opinions hereinafter set forth.

April 11, 2022

Page 2

In such examination, we have assumed the genuineness of all signatures, the legal capacity of all natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as certified, conformed or photostatic copies and the authenticity of the originals of such latter documents. As to all questions of fact material to these opinions that have not been independently established, we have relied upon certificates or comparable documents of officers and representatives of the Company.

With respect to the Warrant Shares and the Selling Securityholder Warrants, we have assumed that the Warrant Agreement and Selling Securityholder Warrants have been duly authorized, executed and delivered by Continental Stock Transfer & Trust Company, as warrant agent (the “Warrant Agent”), and constitute legal, valid and binding obligations of the Warrant Agent, enforceable in accordance with their terms, and we express no opinion to the extent that future issuances of securities of the Company, including the Warrant Shares, and/or anti-dilution adjustments to outstanding securities of the Company, including the Selling Securityholder Warrants, may cause the Selling Securityholder Warrants to be exercisable for more shares of Common Stock than the number that then remain authorized but unissued. We have also assumed that at or prior to the time of the delivery of any of the Warrant Shares, the Registration Statement will have been declared effective under the Act, and no stop orders suspending the Registration Statement’s effectiveness will have been issued and remain in effect.

Based on the foregoing, and subject to the qualifications stated herein, we are of the opinion that:

1. The Warrant Shares have been duly authorized and, when issued and paid for upon exercise of the Warrants in accordance with the terms of the Selling Securityholder Warrants, will be validly issued, fully paid and nonassessable;

2. The Selling Securityholder Warrants constitute legal, valid and legally binding obligations of the Company, enforceable against the Company in accordance with their terms, subject to applicable bankruptcy, insolvency, fraudulent conveyance, reorganization, moratorium and similar laws affecting creditors’ rights and remedies generally, and subject, as to enforceability, to general principles of equity, including principles of commercial reasonableness, good faith and fair dealing (regardless of whether enforcement is sought in a proceeding at law or in equity); and

3. The Selling Securityholder Shares have been duly authorized and are validly issued, fully paid and nonassessable.

April 11, 2022

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The opinions expressed herein are limited to the corporate laws of the State of Delaware and, solely with respect to whether or not the Selling Securityholder Warrants are the legal, valid and legally binding obligations of the Company, the laws of the State of New York, and we express no opinion as to the effect on the matters covered by this letter of the laws of any other jurisdiction.

We hereby consent to the filing of this letter as an exhibit to the Registration Statement and to the reference to our firm under the caption “Legal Matters” in the prospectus which is a part of the Registration Statement. In giving such consent, we do not hereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Securities and Exchange Commission.

Very truly yours,

/s/ Weil, Gotshal & Manges LLP