As filed with the Securities and Exchange Commission on March 23, 2026

Registration No. 333-   

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-8

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Abivax SA

(Exact name of Registrant as specified in its charter)

7-11 boulevard Haussmann 75009 Paris, France

(Address of Registrant’s principal executive offices)

Free share plan AGA 2025-6

Free share plan AGA 2025-7

Free share plan AGA 2025-8

Free share plan AGA 2026-1

Share warrants plan BSA-2026-1

(Full title of the plans)

Marc de Garidel

Chief Executive Officer

7-11 boulevard Haussmann

75009 Paris, France

Tel. No. +33 (0) 1 53 83 09 63

(Name, address and telephone number of agent for service)

Copies to:

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 definitions of “large accelerated filer,” “accelerated filer,” “small 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 provided pursuant to Section 7(a)(2)(B) of the Securities Act. ☒

PART I

INFORMATION REQUIRED IN THE SECTION 10(A) PROSPECTUS

The information required by Part I of Form S-8 is omitted from this filing in accordance with Rule 428 under the Securities Act of 1933, as amended (the “Securities Act”) and the introductory note to Part I of Form S-8. The documents containing the information specified in Part I will be delivered to the participants in the plans covered by this Registration Statement as required by Rule 428(b)(1) under the Securities Act.

PART II

INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3. Incorporation of Documents by Reference.

This Registration Statement on Form S-8 hereby incorporates by reference the contents of the following reports of Abivax SA (the “Registrant” or “we”) filed with, or furnished to, the Securities and Exchange Commission (the “Commission”) pursuant to the Securities Exchange Act of 1934, as amended (the “Exchange Act”):

Notwithstanding the foregoing, no document, report or exhibit (or portion of any of the foregoing) or any other information that we have “furnished” to the Commission pursuant to the Exchange Act shall be incorporated by reference into this prospectus unless such document, report or exhibit expressly provides to the contrary.

Statements contained in the foregoing documents incorporated by reference shall be deemed to be modified or superseded hereby to the extent that statements contained in the Prospectus, or in any subsequently filed documents that are amendments hereto or that are incorporated herein by reference, shall modify or replace such statements.

Item 4. Description of Securities.

Not applicable.

Item 5. Interests of Named Experts and Counsel.

Not applicable.

Item 6. Indemnification of Directors and Officers.

Under French law, provisions of by-laws that limit the liability of directors are prohibited. However, French law allows société anonyme to contract for and maintain liability insurance against civil liabilities incurred by any of their directors and officers involved in a third-party action, provided that they acted in good faith and within their capacities as directors or officers of the company. Criminal liability cannot be indemnified under French law, whether directly by the company or through liability insurance. We maintain liability insurance for our directors and officers, including insurance against liability under the Securities Act.

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Certain of our non-employee directors may, through their relationships with their employers or partnerships, be insured and/or indemnified against certain liabilities in their capacity as members of our board of directors.

Item 7. Exemption from Registration Claimed.

Not applicable.

Item 8. Exhibits.

See the Exhibit Index, below.

Item 9. Undertakings.

The undersigned Registrant hereby undertakes:

provided, however, that paragraphs (i) and (ii) do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in periodic reports filed with or furnished to the Commission by the Registrant pursuant to Section 13 or 15(d) of the Exchange Act that are incorporated by reference in the Registration Statement;

The undersigned Registrant hereby undertakes that, for purposes of determining any liability under the Securities Act, each filing of the Registrant’s annual report pursuant to Section 13(a) or Section 15(d) of the Exchange Act (and, where applicable, each filing of an employee benefit plan’s annual report pursuant to Section 15(d) of the Exchange Act) that is incorporated by reference in the Registration Statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

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Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons of the Registrant pursuant to the foregoing provisions, or otherwise, the Registrant has been advised that in the opinion of the Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the Registrant of expenses incurred or paid by a director, officer or controlling person of the Registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the Registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

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EXHIBIT INDEX

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SIGNATURES

Pursuant to the requirements of the Securities Act, Abivax SA 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 Paris, France on March 23, 2026.

POWER OF ATTORNEY AND SIGNATURES

KNOW ALL BY THESE PRESENTS, that each person whose signature appears below constitutes and appoints Marc de Garidel and Didier Blondel, and each of them, as his or her true and lawful attorney-in-fact and agents, each with the full power of substitution, for him or her and in his or her name, place or stead, in any and all capacities, to sign any and all amendments to this registration statement (including post-effective amendments), and to file the same, with exhibits thereto and other documents in connection therewith, with the Securities and Exchange Commission, granting unto said attorney-in-fact and agents, and each of them, full power and authority to do and perform each and every act and thing requisite and necessary to be done in and about the premises, as fully to all intents and purposes as he or she might or could do in person, hereby ratifying and confirming all that said attorney-in-fact and agents, or their or his substitute or substitutes, 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 has been signed by the following persons in the capacities and on the dates indicated.

SIGNATURE OF AUTHORIZED U.S. REPRESENTATIVE

Pursuant to the Securities Act of 1933, as amended, the undersigned, the duly authorized representative in the United States of Abivax SA, has signed this registration statement on March 23, 2026.

Exhibit 4.4

Summary of BSA Plans

The following description is a summary of the principal features of the following share warrant plans of Abivax SA (the “Company”) having U.S. beneficiaries being registered on the Company’s Registration Statement on Form S-8 to which this description is an exhibit: BSA-2026-1 Plan.

Share warrants (bons de souscription d’actions), or BSAs, entitle a holder to exercise the warrant for the underlying vested shares at an exercise price per share determined by the Company’s board of directors and at least equal to the fair market value of an ordinary share on the date of grant. In addition to any exercise price payable by a holder upon the exercise of any share warrant, share warrants need to be subscribed for at a price which is determined by the board of directors at the time of the grant.

Administration. Pursuant to delegations granted at the Company’s annual meeting, the Company’s board of directors determines the recipients, dates of grant and exercise price of share warrants, the number of share warrants to be granted and the terms and conditions of the share warrants, including their exercise period and their vesting period. In its discretion, the board of directors has the authority to extend the exercise period of share warrants post-termination.

Underlying shares. Each BSA-2026-1 gives the holder the right to purchase one (1) ordinary share.

Allocation. The Company’s BSAs are generally subscribed by officers, directors, employees, consultants or partners of the Company. BSAs 2026-1 may not be transferred.

Standard terms. The conditions to exercise BSAs 2026-1 are as follows:

Validity period. All BSAs have a maximum validity period of 10 years as from their issuance.

Exhibit 4.5

Summary of Free Share Plans

The following description is a summary of the principal features of the following free share plans of Abivax SA (the “Company”) having U.S. beneficiaries being registered on the Company’s Registration on Form S-8 to which this description is an exhibit: AGA-2025-6, AGA-2025-7, AGA-2025-8 and AGA-2026-1.

Free Shares (actions gratuites) are allotted for free to holders. The issuance of the Free Shares occurs automatically at the end of the vesting period (période d’acquisition), subject to the fulfilment of the vesting conditions (if any), by way of a capital increase, which will be realized by debiting the unavailable reserve (réserve non disponible) established for this matter.

Administration. Pursuant to authorizations granted at the Company’s annual meeting, the Company’s board of directors determines the recipients, the number of free shares to be granted and the terms and conditions of the free shares, including their vesting calendar and conditions.

Underlying shares. Each Free Share gives the right to one (1) ordinary share.

Allocation. The Company’s Free Shares are generally granted to executive officers, directors, or employees of the Company or its subsidiaries.

Vesting periods and conditions. The Free Shares vest as follows:

A condition of presence is applicable to all Free Shares 2025-6.

A condition of presence is applicable to all Free Shares 2025-7.

A condition of presence is applicable to all Free Shares 2025-8.

A condition of presence is applicable to all Free Shares 2026-1.

Standard terms.

Pursuant to Article L.225-197-3, if a holder deceases during the vesting period, heirs may request, within six (6) months of the holder’s death, the immediate allocation of the Free Shares for which the condition of presence and/or the applicable performance condition(s), if any, have been met as of the date of the holder’s death.

In the event that the vesting period expires prior to the second anniversary of the grant of the Free Shares, a lock-up period is applicable to the Free Shares until the date of the second anniversary of the grant. Free Shares vesting after the second anniversary of the grant are not subject to lock-up.

Additionally, all Free Shares are subject to a vesting acceleration condition in case of a tender offer on the securities issued by the company and resulting in a change of control of the company.

Exhibit 5.1

				22 rue Bayard 75008 Paris France +33 1 57 57 80 80 Main +33 1 57 57 80 81 Fax Palais J096 www.dechert.com

March 23, 2026

Abivax S.A.

7-11 Boulevard Haussmann

75009 Paris

France

Ladies and Gentlemen:

We have acted as special French counsel to Abivax S.A., a French société anonyme (the “Company”) in connection with the filing of the Registration Statement on Form S-8 (the “Registration Statement”) relating to the registration of up to 989,845 ordinary shares of the Company, par value €0.01 per share (the “Shares”) pursuant to the Company’s BSA-2026-1 Share Warrants Plan, AGA 2025-6 Free Share Plan, AGA 2025-7 Free Share Plan, AGA 2025-8 Free Share Plan, and AGA 2026-1 Free Share Plan (collectively, the “Plans”).

In connection with the opinion expressed herein, we have examined such documents, records and matters of law as we have deemed relevant or necessary for purposes of such opinion. We have assumed the genuineness and authenticity of all documents submitted to us as originals and the conformity to originals of all documents submitted to us as copies thereof. In rendering this opinion, as to certain factual matters, we have, with your consent, relied upon oral and written representations of officers of the Company with respect to the accuracy of the factual matters addressed in such representations.

In such examination, we have assumed that the entry into the Plans and the issuance of the Shares by the Company are (a) in the Company’s corporate interest and (b) serving the Company’s corporate purpose (objet social) as set forth in its statuts (bylaws) or other constitutional documents.

The opinion expressed herein is limited to the laws of France as currently in effect, and we express no opinion as to the effect of the laws of any other jurisdiction. We have assumed that the Company will take no action inconsistent with the Plans and the resolutions authorizing the Company to issue the Shares. We have also assumed that (1) the resolutions authorizing the Company to issue the Shares pursuant to the respective Plans as adopted by the extraordinary shareholders’ meetings, the Board of Directors (the “Board”) and, the chief executive officer (directeur général) of the Company, if applicable, have not been or will not be amended or superseded, have been or will be duly passed at duly convened and held meetings and, with respect to the Board, of duly appointed members, (2) the resolutions authorizing the Company to issue the Shares pursuant to the respective Plans as adopted by the shareholders’ meeting, the Board and the chief executive officer (directeur général) of the Company, if applicable, and the applicable award agreements have been or will be in full force and effect on the date of such awards, (3) for any future awards under the Plans, such future awards will be approved by the Board and, if applicable, the chief executive officer (directeur général) of the Company in accordance with applicable law, the resolutions of the shareholders’ meeting and with the terms of the relevant Plan, (4) the amount of Shares awarded under the Plans have been and will remain within the limits of the then authorized but unissued amounts of Shares pursuant to the resolutions set forth in (2) above, and (5) all Shares issued under the Plans will be issued in compliance with applicable law and with the relevant terms of the Plans.

Dechert (Paris) LLP est un limited liability partnership immatriculé en Angleterre et au Pays de Galles (No. d’immatriculation OC332363), autorisé et régi par la Solicitors Regulation Authority, inscrit au Barreau de Paris en application de la Directive 98/5/CE. La liste des membres et non-membres de Dechert (Paris) LLP désignés comme associés (qui sont solicitors ou registered foreign lawyers et/ou membres du Barreau de Paris) peut être consultée à l’adresse ci-dessous à notre siège sis: 160 Queen Victoria Street, Londres EC4V 4QQ, Royaume-Uni.

Membre d’une association agréée, le règlement des honoraires par chèque ou virement bancaire est accepté.

Dechert exerce en tant que limited liability partnership ou limited liability company hors Dublin et Hong Kong.

Based on the foregoing, and subject to the further limitations, qualifications and assumptions set forth herein, we are of the opinion that the Shares that may be issued pursuant to the Plans have been duly authorized by the Shareholders’ Meeting and, when issued in accordance with the respective Plans and against payment of due consideration therefor (to the extent applicable), will be validly issued, fully paid and non-assessable.

The term “non-assessable”, which has no recognized meaning in French law, for the purposes of this opinion means that no present or future holder of Shares issued under the Plans will be subject to personal liability, by reason of being such a holder, for additional payments or calls for further funds by the Company or any other person after the issuance of the Shares issued under the Plans.

We do not undertake or accept any obligation to update this opinion to reflect subsequent changes in French law or factual matters arising after the date of effectiveness of this Registration Statement.

The opinion set out above is subject to the following qualifications:

We are members of the Paris bar and this opinion is limited to the laws of the Republic of France. This opinion is subject to the sovereign power of the French courts to interpret agreements and assess the facts and circumstances of any adjudication. This opinion is given on the basis that it is to be governed by, and construed in accordance with the laws of the Republic of France.

We hereby consent to the filing of this opinion as Exhibit 5.1 to the Registration Statement filed by the Company to effect registration of the Shares to be issued and sold pursuant to each Plan under the Securities Act of 1933, as amended (the “Act”). In giving such consent, we do not thereby admit that we are included in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the U.S. Securities and Exchange Commission promulgated thereunder.

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Exhibit 23.1

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We hereby consent to the incorporation by reference in this Registration Statement on Form S-8 of Abivax SA of our report dated March 23, 2026, relating to the financial statements, which appears in Abivax SA’s Annual Report on Form 20-F for the year ended December 31, 2025.

/s/ PricewaterhouseCoopers Audit

Neuilly-sur-Seine, France

March 23, 2026

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