Our File No.:	58123V-300405
Date:	September 4, 2025

ProMIS Neurosciences Inc.

Suite 200, 1920 Yonge Street

Toronto, Ontario

M4S 3E2

Attention: Board of Directors

Re:

ProMIS Neurosciences Inc.

Resale Registration Statement on Form S-3‎‎

We have acted as Ontario legal counsel to ProMIS Neurosciences Inc. (the “Corporation”) with respect to certain legal matters in connection with the registration statement on Form S-3 filed on the date hereof (the “Registration Statement”) with the Securities and Exchange Commission (the “Commission”), pursuant to the U.S Securities Act of 1933, as amended (the “Securities Act”), relating to the registration by the Corporation under the Securities Act of the resale from time to time by the selling shareholders identified in the ‎Registration Statement (the “Selling Securityholders”) of an aggregate of 28,233,181 common shares, no par value (the “Common Shares”), of the Corporation comprised of:‎ ‎

(a)

12,616,821 Common Shares (the “July 22 Warrant Shares”) issuable immediately upon the exercise of 12,616,821 Common Share purchase warrants (the “July 22 Warrants”) at an exercise price of US$1.25 per July 22 Warrant Share, issued pursuant to a securities purchase agreement dated as of July 22, 2025 between the Corporation and the Selling Securityholder party thereto (the “July 22 Purchase Agreement”), in connection with a private placement offering (the “July 22 Private Placement”), which are exercisable until July 24, 2030; and

(b)

15,616,360 Common Shares (the “July 28 Warrant Shares” and, together with the July 22 Warrant Shares, the “Warrant Shares”) issuable immediately upon the exercise of 15,616,360 Common Share purchase warrants (the “July 28 Warrants” and, together with the July 22 Warrants, the “Warrants”) at an exercise price of US$1.25 per Common Share, which are issued pursuant to a securities purchase agreement dated as of July 28, 2025 between the Corporation and the Selling Securityholders party thereto (the “July 28 Purchase Agreement” and, together with the July 22 Purchase Agreement, the “Purchase Agreements”), in connection with a private placement offering (the “July 28 Private Placement” and, together with the July 22 Private Placement, the “Private Placements”), and are exercisable until July 29, 2030.

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The Warrant Shares are being registered pursuant to Registration Rights Agreements dated July 22, 2025 and July 28, 2025, entered into between the Corporation and the Selling Securityholders (the “Registration Rights Agreements”), pursuant to which the Corporation is required to prepare and file a registration statement with the Commission under the Securities Act, covering the resale of the Warrant Shares issued to the Selling Securityholders under the Purchase Agreements.

All capitalized terms not defined in this opinion have the meaning attributable to them in the Registration Statement.

Scope of Review and Reliances

‎For the purpose of rendering our opinions expressed below, we have examined and relied ‎upon copies of the following ‎documents:‎

(a)

the Purchase Agreements;

(b)

the Registration Rights Agreements;

(c)

the Registration Statement and the exhibits thereto;

(d)

the articles and bylaws of the Corporation in effect as of the date hereof (the “Constating Documents”);

(e)

the forms of certificates representing the July 22 Warrants (the “July 22 Warrant Certificate”) and the July 28 Warrants (the “July 28 Warrant Certificates”);

(f)

a certificate of the Chief Executive Officer of the Corporation as to certain factual matters relating to the Private Placements dated the date hereof (the “Officer’s Certificate”); and

(g)

such other documents as we have deemed relevant.

We have relied exclusively upon the certificates, documents and records referred to above with respect to the accuracy of the factual matters contained therein and we have not performed any independent investigation or verification of such factual matters.

We have considered such questions of law, made such investigations and examined and relied upon ‎such originals, facsimiles or copies, certified or otherwise identified to our satisfaction, of such additional ‎public and corporate records, records of corporate proceedings, certificates and other documents as we ‎have considered relevant or necessary in order to render our opinions expressed herein.‎

Our opinion expressed herein is subject in all respects to the following assumptions, limitations and qualifications:

(a)

We are solicitors qualified to practice law in the Province of Ontario. This opinion is limited to the laws of the Province of Ontario, including all applicable provisions of the Business Corporations Act (Ontario) (the “OBCA”) and the federal laws of Canada applicable in Ontario, in each case as in effect on the date hereof. We have not considered, and have not expressed any opinion with regard to, or as to the effect of, any other law, rule or regulation, state or federal, applicable to the Corporation. In particular, we express no opinion as to United States federal securities laws. We have no responsibility and assume no responsibility to advise you of any changes to applicable laws or any changes involving the Corporation after the date hereof;

(b)We have assumed: (i) the genuineness of all signatures on documents examined by us, including electronic signatures; (ii) the legal capacity of the officers of the Corporation; (iii) the authenticity of all documents submitted to us as originals; (iv) the conformity to authentic originals of all documents submitted to us as certified, conformed, photostatic or other copies;


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	Page 3

and (v) that the documents, in the forms submitted to us for our review, have not been and will not be altered or amended in any respect;

(c)We have assumed that each of the statements made and certified in the Officer’s Certificate was true and correct when made and will continue to be true and correct;

(d)We have assumed at the time of issuance of any Warrant Shares the Corporation ‎will be validly existing under the OBCA and will have the necessary corporate power for such issuance; and

(e)We have assumed at the time of issuance of any Warrant Shares, the Constating Documents are in full force ‎‎and effect and have not been amended, restated, supplemented or otherwise ‎altered in such a matter to prohibit such issuance and there has ‎been no authorization of any such amendment, ‎restatement, supplement or other alteration, in either ‎case since the date hereof.

Opinions

Based and relying on the foregoing, we are of the opinion that:

The issuance of the 12,616,821 July 22 Warrant Shares upon due exercise of the July 22 Warrants have been duly authorized by the Corporation and upon exercise of such July 22 Warrants in accordance ‎with the terms of the July 22 Warrant Certificate, including full payment of the exercise price per July 22 Warrant Share ‎purchased, the July 22 Warrant Shares will be duly and ‎validly issued as fully paid and non-assessable Common Shares in the capital of the Corporation.

2.The issuance of the 15,616,360 July 28 Warrant Shares upon due exercise of the July 28 Warrants have been duly authorized by the Corporation and upon exercise of such July 28 Warrants in accordance ‎with the terms of the July 28 Warrant Certificates, including full payment of the exercise price per July 28 Warrant Share ‎purchased, the July 28 Warrant Shares will be duly and ‎validly issued as fully paid and non-assessable Common Shares in the capital of the Corporation.

Our opinions are rendered solely for the benefit of the addressees, are being delivered in connection with the transactions described herein and may not be relied upon by any other person or in connection with any other transaction, quoted from or referred to in any other documents, or furnished (either in its original form or by copy) to any other person without our prior written consent.

We consent to the use of this opinion as an exhibit to the Registration Statement and further consent to the use of our name wherever appearing in the Registration Statement and in any amendment thereto.

Yours Truly,

“McMillan LLP”