EX-5.1

Exhibit 5.1

Harney Westwood & Riegels

3501 The Center

99 Queen’s Road Central

Hong Kong

Tel: +852 5806 7800

Fax: +852 5806 7810

2 May 2025

052680.0032

I-Mab

2440 Research Boulevard, Suite 400

Rockville, MD 20850

United States

Dear Sir or Madam

I-Mab (the Company)

We are lawyers qualified to practise in the Cayman Islands and have acted as Cayman Islands legal advisers to the Company in connection with a registration statement on form F-3, including all amendments or supplements thereto (the Registration Statement, which term does not include any other document or agreement whether or not specifically referred to therein or attached as an exhibit or schedule thereto), filed with the U.S. Securities and Exchange Commission (the Commission) on or about the date hereof relating to the registration under the U.S. Securities Act of 1933, (the Securities Act), as amended to date, relating to the offering and sale of up to $250,000,000 ordinary shares of a par value of US$0.0001 each in the share capital of the Company (the Ordinary Shares) and American Depositary Shares (the ADSs) , each ten ADSs representing twenty-three (23) Ordinary Shares (together, the Offer Shares), including up to $21,000,000 in the aggregate of ADSs that may be issued and sold under the Sales Agreement (as defined in Schedule 1). In this opinion Companies Act means the Companies Act (Revised) of the Cayman Islands.

We are furnishing this opinion as Exhibit 5.1 to the Registration Statement.

For the purposes of giving this opinion, we have examined the Documents (as defined in Schedule 1). We have not examined any other documents, official or corporate records or external or internal registers and have not undertaken or been instructed to undertake any further enquiry or due diligence in relation to the transaction which is the subject of this opinion.

In giving this opinion we have relied upon the assumptions set out in Schedule 2 which we have not independently verified.

Based solely upon the foregoing examinations and assumptions and upon such searches as we have conducted and having regard to legal considerations which we deem relevant, and subject to the qualifications set out in Schedule 3, we are of the opinion that under the laws of the Cayman Islands:

The Company is duly incorporated and validly existing as an exempted company with limited liability under the law of the Cayman Islands and, based on the Certificate of Good Standing (as defined in Schedule 1), is in good standing as at 22 April 2025. Pursuant to the Companies Act, a company is deemed to be in good standing if all fees and penalties under the Companies Act have been paid and the Registrar of Companies has no knowledge that the Company is in default under the Companies Act.

The British Virgin Islands is Harneys Hong Kong office’s main jurisdiction of practice. Jersey legal services are provided through a referral arrangement with Harneys (Jersey) which is an independently owned and controlled Jersey law firm.

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2.	The authorised share capital of the Company is US$80,000 divided into 800,000,000 ordinary shares of a par value of US$0.0001 each.

The allotment and issue of Offer Shares has been duly authorised, and when allotted, issued and paid for as contemplated in the Registration Statement and, as applicable, the Sales Agreement, the Offer Shares will be validly issued, fully paid and non-assessable (which term means when used herein that no further sums are required to be paid by the holders thereof in connection with the issue of such shares) upon (i) payment in full of the consideration as set out in the Registration Statement and in accordance with the terms set out in the Registration Statement and in accordance with the M&A, and (ii) the entry of the Offer Shares as fully paid on the register of members of the Company,.

This opinion is confined to the matters expressly opined on herein and given on the basis of the laws of the Cayman Islands as they are in force and applied by the Cayman Islands courts at the date of this opinion. We have made no investigation of, and express no opinion on, the laws of any other jurisdiction. Except as specifically stated herein, we express no opinion as to matters of fact.

In connection with the above opinion, we hereby consent to the filing of this opinion as an exhibit to the Registration Statement, which is incorporated by reference into the Registration Statement. In giving such consent, we do not thereby admit that we come within the category of persons whose consent is required under Section 7 of the Securities Act, as amended or the Rules and Regulations of the Commission thereunder.

This opinion is limited to the matters referred to herein and shall not be construed as extending to any other matter or document not referred to herein.

This opinion shall be construed in accordance with the laws of the Cayman Islands.

Yours faithfully

/s/ Harney Westwood & Riegels

Harney Westwood & Riegels

SCHEDULE 1

List of Documents and Records Examined

1	A copy of the certificate of incorporation of the Company dated 30 June 2016.

2	A copy of the certificate of incorporation on adoption of dual foreign name of the Company dated 27 June 2018.

A copy of the sixth amended and restated memorandum and articles of association of the Company adopted by a special resolution passed on 29 October 2019 and effective immediately prior to the completion of the initial public offering of the Company’s American Depositary Shares representing its Ordinary Shares (the M&A).

4	A copy of the register of directors and officers of the Company provided to us on 23 April 2025.

5	A certificate of good standing in respect of the Company dated 22 April 2025.

Copies of 1 to 5 above have been provided to us by the Company (the Corporate Documents, and together with 6 to 9 below, the Documents).

6	A copy of the executed written resolutions of the board of directors of the Company in April 2025 (the *Resolutions*).

7	A copy of the sales agreement dated 2 May 2025 and entered into by and between the Company and Leerink Partners LLC (the *Sales Agreement*).

8	A certificate issued by 23 April 2025, being a director of the Company dated Sean Fu, a copy of which is attached hereto (the *Director’s Certificate*).

9	The Registration Statement.

SCHEDULE 2

Assumptions

Authenticity of Documents. Copy documents or drafts of documents provided to us are true and complete copies of, or in the final forms of, the originals. All original Corporate Documents are authentic, all signatures, initials and seals are genuine, all copies of the Registration Statement and the Sales Agreement are true and correct copies and the Registration Statement and the Sales Agreement conform in every material respect to the latest drafts of the same produced to us and, where the Registration Statement and the Sales Agreement have been provided to us in successive drafts marked-up to indicate changes to such documents, all such changes have been so indicated.

Corporate Documents. All matters required by law to be recorded in the Corporate Documents are so recorded, and all corporate minutes, resolutions, certificates, documents and records which we have reviewed are accurate and complete, and all facts expressed in or implied thereby are accurate and complete as at the date of the passing of the Resolutions.

3	No Steps to Wind-up. The directors and shareholders of the Company have not taken any steps to appoint a liquidator of the Company and no receiver has been appointed over any of the Company’s property or assets.

Resolutions. The Resolutions have been duly executed (and where by a corporate entity such execution has been duly authorised if so required) by or on behalf of each director, or by or on behalf of each shareholder in respect of the shareholder resolutions, and the signatures and initials thereon are those of a person or persons in whose name the Resolutions have been expressed to be signed. The Resolutions remain in full force and effect.

Unseen Documents. Save for the Documents provided to us there are no resolutions, agreements, documents or arrangements which materially affect, amend or vary the transactions envisaged in the Documents and, in particular, that the entry into and performance of the transactions contemplated under the Registration Statement and the Sales Agreement will not cause any of the parties thereto to be in breach of any agreement or undertaking.

6	Constitutional Documents. The M&A is the latest memorandum and articles of association of the Company in effect as of the time of the opinion.

7	Director’s Certificate. The contents of the Director’s Certificate are true and accurate as at the date of this opinion and there is no information not contained in the Director’s Certificate that will in any way affect this opinion.

SCHEDULE 3

Qualifications

1	Foreign Statutes. We express no opinion in relation to provisions making reference to foreign statutes in the Registration Statement and the Sales Agreement.

Commercial Terms. Except as specifically stated herein, we make no comment with respect to any representations and warranties which may be made by or with respect to the Company in any of the documents or instruments cited in this opinion or otherwise with respect to the commercial terms of the transactions the subject of this opinion.

Non-assessable. In this opinion the phrase non-assessable means, with respect to the issuance of shares, that a shareholder shall not, in respect of the relevant shares, have any obligation to make further contributions to the Company’s assets (except in exceptional circumstances, such as involving fraud, the establishment of an agency relationship or an illegal or improper purpose or other circumstances in which a court may be prepared to pierce or lift the corporate veil).

Register of members. Under the Companies Act, the register of members of a Cayman Islands company is by statute regarded as prima facie evidence of any matters which the Companies Act directs or authorises to be inserted therein. A third party interest in the shares in question would not appear. An entry in the register of members may yield to a court order for rectification (for example, in the event of fraud or manifest error).

Good Standing. The Company shall be deemed to be in good standing at any time if all fees (including annual filing fees) and penalties under the Companies Act have been paid and the Registrar of Companies has no knowledge that the Company is in default under the Companies Act (Revised) of the Cayman Islands.

6	Economic Substance. We have undertaken no enquiry and express no view as to the compliance of the Company with the International Tax Co-operation (Economic Substance) Act (Revised).