Exhibit 5.1

ATTORNEYS • CIVIL LAW NOTARIES • TAX ADVISERS

P.O. Box 7113
1007 JC Amsterdam
Beethovenstraat 400
1082 PR Amsterdam
T +31 20 71 71 000		Amsterdam, 29 September 2025.
F +31 20 71 71 111

To the Company:

We have acted as legal counsel as to Dutch law to the Company in connection with the filing of the Registration Statement with the SEC. This opinion letter is rendered to you in order to be filed with the SEC as an exhibit to the Registration Statement.

Capitalised terms used in this opinion letter have the meanings set forth in Exhibit A to this opinion letter. The section headings used in this opinion letter are for convenience of reference only and are not to affect its construction or to be taken into consideration in its interpretation.

This opinion letter is strictly limited to the matters stated in it and may not be read as extending by implication to any matters not specifically referred to in it. Nothing in this opinion letter should be taken as expressing an opinion in respect of any representations or warranties, or other information, contained in any document reviewed by us in connection with this opinion letter.

In rendering the opinions expressed in this opinion letter, we have reviewed and relied upon a draft of the Registration Statement, pdf copies of the Corporate Documents and pdf copies the Reviewed Documents, and we have assumed that any issuance of Registered Securities and the Resale Shares has been or shall be effected for bona fide commercial reasons. We have not investigated or verified any factual matter disclosed to us in the course of our review.

This opinion letter sets out our opinion on certain matters of the laws with general applicability of the Netherlands, and, insofar as they are directly applicable in the Netherlands, of the European Union, as at today's date and as presently interpreted under published authoritative case law of the Dutch courts, the General Court and the Court of Justice of the European Union. We do not express any opinion on Dutch or European competition law, data protection law, tax law, securitisation law or regulatory law. No undertaking is assumed on our part to revise, update or amend this opinion letter in connection with, or to notify or inform you of, any developments and/or changes of Dutch law subsequent to today's date. We do not purport to opine on the consequences of amendments to the Registration Statement, the Corporate Documents or Reviewed Documents subsequent to the date of this opinion letter.

Amsterdam	All legal relationships are subject to NautaDutilh N.V.'s general terms and conditions (see https://www.nautadutilh.com/terms), which apply mutatis mutandis to our relationship with third parties relying on statements of NautaDutilh N.V., include a limitation of liability clause, have been filed with the Rotterdam District Court and will be provided free of charge upon request. NautaDutilh N.V.; corporate seat Rotterdam; trade register no. 24338323.

Brussels

London

Luxemburg

New York

Rotterdam



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The opinions expressed in this opinion letter are to be construed and interpreted in accordance with Dutch law. The competent courts at Amsterdam, the Netherlands, have exclusive jurisdiction to settle any issues of interpretation or liability arising out of or in connection with this opinion letter. Any legal relationship arising out of or in connection with this opinion letter (whether contractual or non-contractual), including the above submission to jurisdiction, is governed by Dutch law and shall be subject to the general terms and conditions of NautaDutilh. Any liability arising out of or in connection with this opinion letter shall be limited to the amount which is paid out under NautaDutilh's insurance policy in the matter concerned. No person other than NautaDutilh may be held liable in connection with this opinion letter.

In this opinion letter, legal concepts are expressed in English terms. The Dutch legal concepts concerned may not be identical in meaning to the concepts described by the English terms as they exist under the law of other jurisdictions. In the event of a conflict or inconsistency, the relevant expression shall be deemed to refer only to the Dutch legal concepts described by the English terms.

For the purposes of this opinion letter, we have assumed that:

a.	each copy of a document conforms to the original, each original is authentic, and each signature is the genuine signature of the individual purported to have placed that signature;

b.	if any signature under any document is an electronic signature (as opposed to a handwritten ("wet ink") signature) only, it is either a qualified electronic signature within the meaning of the eIDAS Regulation, or the method used for signing is otherwise sufficiently reliable;

c.	the Registration Statement has been or will be declared effective by the SEC in the form reviewed by us;

at each Relevant Moment, (i) Ordinary Shares have been and shall have been, as applicable, admitted for trading on a trading system outside the European Economic Area comparable to a regulated market or a multilateral trading facility as referred to in Section 2:86c(1) DCC and (ii) no financial instruments issued by the Company (or depository receipts for or otherwise representing such financial instruments) have been or shall have been, as applicable, admitted to trading on a regulated market, multilateral trading facility or organised trading facility operating in the European Economic Area (and no request for admission of any such financial instruments to trading on any such trading venue has been or shall have been, as applicable, made);

e.	(i) the Previous Articles were the Articles of Association in force at the Relevant Moment of issuance of Resale Shares pursuant to the June Deeds of Issue, and (ii) the Current Articles were and shall be, as applicable, the Articles of Association in force at each other Relevant Moment;



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f.	each of the Deed of Incorporation, the Deed of Conversion, the Previous Deed of Amendment and the Deed of Amendment is a valid notarial deed;

g.	at each Relevant Moment, the authorised share capital (maatschappelijk kapitaal) of the Company allowed or shall allow, as applicable, the issuance of the Resale Shares, Registered Shares and other Registered Securities;

at each Relevant Moment, the Company had not, and shall not have, as applicable, (i) been dissolved (ontbonden), (ii) ceased to exist pursuant to a merger (fusie) or a division (splitsing), (iii) been converted (omgezet) into another legal form, either national or foreign (except pursuant to the Deed of Conversion), (iv) had its assets placed under administration (onder bewind gesteld), (v) been declared bankrupt (failliet verklaard), (vi) been granted a suspension of payments (surseance van betaling verleend), (vii) started or become subject to statutory proceedings for the restructuring of its debts (akkoordprocedure) or (viii) been made subject to similar proceedings in any jurisdiction or otherwise been limited in its power to dispose of its assets;

the Resale Shares, Registered Shares and other Registered Securities have been or shall have been, as applicable, issued and any pre-emption rights in connection therewith have been or shall have been, as applicable, excluded, pursuant to resolutions validly passed by the corporate body (orgaan) of the Company duly authorised to do so;

(i) the issue price for the Resale Shares and the Registered Shares equaled or shall equal, as applicable, at least the aggregate nominal value thereof, (ii) any pre-funded purchase price, conversion price, exchange price or exercise price, as applicable, of the Convertible Notes, Pre-Funded Warrants and Registered Securities equaled or shall equal, as applicable, at least the aggregate nominal value of the underlying Resale Shares or Registered Shares, as applicable, (iii) any such pre-funded purchase price shall be considered to have been pre-paid on the relevant Resale Shares underlying the Pre-Funded Warrants, (iv) any such issue, pre-funded purchase price, conversion price, exchange price or exercise price has been or shall have been, as applicable, satisfied in cash or by means of a set-off (verrekening) and has been or shall have been, as applicable, received and accepted by the Company ultimately upon the issuance of the relevant Resale Shares and Registered Shares, as applicable, and (v) where relevant, the Company has consented or shall have consented, as applicable, to payment in a currency other than Euro;



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in respect of any payment (storting) on any Resale Shares or Registered Shares satisfied or to be satisfied, as applicable, by means of a set-off (verrekening), the Company has or shall have, as applicable, validly effected such set-off of a debt of the Company vis-à-vis the relevant subscriber for such Resale Shares or Registered Shares, as applicable, against the Company's claim on such subscriber to pay up such Resale Shares or Registered Shares, as applicable, in full in accordance with Section 6:127 DCC, the Company had or shall have, as applicable, the authority to set off such debt vis-à-vis such claim and the amount payable by the Company under such debt was or shall be, as applicable, sufficient to pay up such Resale Shares or Registered Shares, as applicable, in full by means of such set-off, in each case at the moment when the relevant Resale Shares or Registered Shares, as applicable, were or are, as applicable, issued;

any Resale Shares and Registered Shares issued or to be issued, as applicable, in connection with the conversion, exchange or exercise of Convertible Notes, Pre-Funded Warrants or other Registered Securities shall be issued pursuant to a valid conversion, exchange or exercise of such Convertible Notes, Pre-Funded Warrants or Registered Securities, as applicable, in accordance with their respective terms;

the Convertible Notes, Pre-Funded Warrants and Registered Securities (other than Registered Shares) (i) have been validly granted as rights to subscribe for Ordinary Shares (recht tot het nemen van aandelen), (ii) shall be in full force and effect upon being converted, exercised or exchanged, as applicable, and (iii) shall have been validly converted, exercised or exchanged, as applicable, in accordance with the terms thereof;

n.	all Reviewed Documents are, have always been, and at each Relevant Moment after the date of this opinion letter shall be valid, binding and enforceable in accordance with their terms and have been duly and validly signed, executed and delivered by the respective parties thereto;

no Resale Shares, Convertible Notes, Pre-Funded Warrants, Registered Shares or other Registered Securities have been or shall be, as applicable, offered to the public (aanbieden aan het publiek) in the Netherlands other than in conformity with the Prospectus Regulation, the PRIIPs Regulation and the rules promulgated thereunder.

Based upon and subject to the foregoing and subject to the qualifications set forth in this opinion letter and to any matters, documents or events not disclosed to us, we express the following opinions:

Corporate Status

1.	The Company has been duly incorporated as a besloten vennootschap met beperkte aansprakelijkheid and is validly existing as a naamloze vennootschap.



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Registered Shares

Subject to receipt by the Company of payment in full for, or other satisfaction of the issue, conversion, exchange or exercise price for, the Registered Shares, and when issued by the Company and accepted by the acquiror(s) thereof, the Registered Shares shall be validly issued, fully paid and non-assessable.

Resale Shares

3.	The existing Resale Shares have been validly issued and are fully paid and non-assessable.

Subject to receipt by the Company of payment in full for, or other satisfaction of the issue, conversion, exchange or exercise price for, the Resale Shares not yet existing, and when issued by the Company and accepted by the acquiror(s) thereof, those Resale Shares shall be validly issued, fully paid and non-assessable.

The opinions expressed above are subject to the following qualifications:

Opinion 1 must not be read to imply that the Company cannot be dissolved (ontbonden). A company such as the Company may be dissolved, inter alia by the competent court at the request of the company's board of directors, any interested party (belanghebbende) or the public prosecution office in certain circumstances, such as when there are certain defects in the incorporation of the company. Any such dissolution will not have retro-active effect.

Pursuant to Section 2:7 DCC, any transaction entered into by a legal entity may be nullified by the legal entity itself or its liquidator in bankruptcy proceedings (curator) if the objects of that entity were transgressed by the transaction and the other party to the transaction knew or should have known this without independent investigation (wist of zonder eigen onderzoek moest weten). The Dutch Supreme Court (Hoge Raad der Nederlanden) has ruled that in determining whether the objects of a legal entity are transgressed, not only the description of the objects in that legal entity's articles of association (statuten) is decisive, but all (relevant) circumstances must be taken into account, in particular whether the interests of the legal entity were served by the transaction. Based on the objects clause contained in the Previous Articles and the Current Articles, we have no reason to believe that, by issuing Registered Securities or the Resale Shares, the Company transgressed or would transgress, as applicable, the description of the objects contained in its Articles of Association. However, we cannot assess whether there are other relevant circumstances that must be taken into account, in particular whether the interests of the Company are served by issuing Registered Securities since this is a matter of fact.



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Pursuant to Section 2:98c DCC, a company such as the Company may grant loans (leningen verstrekken) only in accordance with the restrictions set out in Section 2:98c DCC, and may not provide security (zekerheid stellen), give a price guarantee (koersgarantie geven) or otherwise bind itself, whether jointly and severally or otherwise with or for third parties (zich op andere wijze sterk maken of zich hoofdelijk of anderszins naast of voor anderen verbinden) with a view to (met het oog op) the subscription or acquisition by third parties of shares in its share capital or depository receipts. This prohibition also applies to its subsidiaries (dochtervennootschappen). It is generally assumed that a transaction entered into in violation of Section 2:98c DCC is null and void (nietig).

D.	The opinions expressed in this opinion letter may be limited or affected by:

rules relating to Insolvency Proceedings or similar proceedings under a foreign law and other rules affecting creditors' rights generally;

the provisions of fraudulent preference and fraudulent conveyance (Actio Pauliana) and similar rights available in other jurisdictions to insolvency practitioners and insolvency office holders in bankruptcy proceedings or creditors;

claims based on tort (onrechtmatige daad);

sanctions and measures, including but not limited to those concerning export control, pursuant to European Union regulations, under the Dutch Sanctions Act 1977 (Sanctiewet 1977) or other legislation;

the Anti-Boycott Regulation, Anti Money Laundering Laws and related legislation;

any intervention, recovery or resolution measure by any regulatory or other authority or governmental body in relation to financial enterprises or their affiliated entities; and

the rules of force majeure (niet toerekenbare tekortkoming), reasonableness and fairness (redelijkheid en billijkheid), suspension (opschorting), dissolution (ontbinding), unforeseen circumstances (onvoorziene omstandigheden) and vitiated consent (i.e., duress (bedreiging), fraud (bedrog), abuse of circumstances (misbruik van omstandigheden) and error (dwaling)) or a difference of intention (wil) and declaration (verklaring).



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The term "non-assessable" has no equivalent in the Dutch language and for purposes of this opinion letter such term should be interpreted to mean that a holder of an Ordinary Share shall not by reason of merely being such a holder be subject to assessment or calls by the Company or its creditors for further payment on such Ordinary Share.

F.	This opinion letter does not purport to express any opinion or view on the operational rules and procedures of any clearing or settlement system or agency.

We consent to the filing of this opinion letter as an exhibit to the Registration Statement and also consent to the reference to NautaDutilh in the Registration Statement under the caption "Legal Matters". In giving this consent we do not admit or imply that we are a person whose consent is required under Section 7 of the United States Securities Act of 1933, as amended, or any rules and regulations promulgated thereunder.

Sincerely yours,

/s/ NautaDutilh N.V.

NautaDutilh N.V.



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EXHIBIT A

LIST OF DEFINITIONS

"Anti Money Laundering Laws"	The European Anti-Money Laundering Directives, as implemented in the Netherlands in the Money Laundering and Terrorist Financing Prevention Act (Wet ter voorkoming van witwassen en financieren van terrorisme) and the Dutch Criminal Code (Wetboek van Strafrecht).

"Anti-Boycott Regulation"	The Council Regulation (EC) No 2271/96 of 22 November 1996 on protecting against the effects of the extra-territorial application of legislation adopted by a third country, and actions based thereon or resulting therefrom.

"Articles of Association"	The Company's articles of association (statuten) as they read from time to time.

"Bankruptcy Code"	The Dutch Bankruptcy Code (Faillissementswet).

"Commercial Register"	The Dutch Commercial Register (handelsregister).

"Company"	ATAI Life Sciences N.V., a public company with limited liability (naamloze vennootschap), registered with the Commercial Register under number 80299776.

"Convertible Notes"	The following convertible loan notes: a. the 147,950 convertible loan notes evidenced by a ‘form of note certificate’ dated April 11, 2024, issued by the Company pursuant to the signed subscription agreement for convertible loan notes, dated April 11, 2024, between the Company and Aperion Investment Group Limited; and



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	b. the 238,669 convertible loan notes evidenced by a ‘form of note certificate’ issued pursuant to the signed subscription agreement for convertible loan notes dated December 15, 2025, between the Company and Wilde Ventures GmbH.

"Corporate Documents"	The Deed of Incorporation, the Deed of Conversion, the Deed of Amendment, the Deed of Previous Articles and the Current Articles.

"Current Articles"	The Articles of Association as they read as a consequence of the execution of the Deed of Amendment.

"DCC"	The Dutch Civil Code (Burgerlijk Wetboek).

"Debt Securities"	One or more series of debt securities issuable by the Company and registered pursuant to the Registration Statement.

"Deed of Amendment"	The deed of amendment to the Articles of Association dated June 25, 2025.

"Deed of Conversion"	The deed of conversion and amendment to the Articles of Association dated June 18, 2021.

"Deed of Incorporation"	The Company's deed of incorporation (akte van oprichting) dated September 10, 2020.

"Deeds of Issue"	The June Deeds of Issue and the August Deed of Issue.

"eIDAS Regulation"	Regulation (EU) No 910/2014 of the European Parliament and of the Council of 23 July 2014 on electronic identification and trust services for electronic transactions in the internal market and repealing Directive 1999/93/EC.

"Insolvency Proceedings"	Any insolvency proceedings within the meaning of Regulation (EU) 2015/848 of the European Parliament and of the Council of 20 May 2015 on insolvency proceedings (recast), as amended by Regulation (EU) 2021/2260 of the European Parliament and of the Counsel of 15 December 2021, listed in Annex A thereto and any statutory proceedings for the restructuring of debts (akkoordprocedure) pursuant to the Bankruptcy Code.



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"NautaDutilh"	NautaDutilh N.V.

"the Netherlands"	The European territory of the Kingdom of the Netherlands and "Dutch" is in or from the Netherlands.

"Ordinary Shares"	Ordinary shares in the Company's capital, with a nominal value of EUR 0.10 each.

"PIPE Shares"	The Ordinary Shares issued pursuant to: a. the signed private deed of issue of 5,434,782 Ordinary Shares, dated June 3, 2025; b. the signed private deed of issue of 4,558,559 Ordinary Shares, dated June 4, 2025 (together with the signed private deed of issue referenced under a. above, the "June Deeds of Issue"); and c. the signed private deed of issue of 18,264,840 Ordinary Shares, dated August 14, 2025.

"Pre-Funded Warrants"	The right to subscribe for up to: a. 6,311,006 Ordinary Shares defined as "Warrant" in the signed pre-funded warrant entered into between the Company and Ferring Ventures SA, dated June 3, 2025; and b. 4,566,210 Ordinary Shares defined as "Warrant" in the signed pre-funded warrant entered into between the Company and Ferring Ventures SA, dated August 14, 2025.

"Previous Articles"	The Articles of Association contained in the Previous Deed of Amendment.

"Previous Deed of Amendment"	The deed of amendment to the Articles of Association dated July 1, 2022.



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"PRIIPs Regulation"	Regulation (EU) No 1286/2014 of the European Parliament and of the Council of 26 November 2014 on key information documents for packaged retail and insurance-based investment products (PRIIPs).

"Prospectus Regulation"	Regulation (EU) 2017/1129 of the European Parliament and of the Council of 14 June 2017 on the prospectus to be published when securities are offered to the public or admitted to trading on a regulated market, and repealing Directive 2003/71/EC.

"Registered Securities"	The Debt Securities, Registered Shares, Units and Warrants.

"Registered Shares"	The following Ordinary Shares: a. the Ordinary Shares registered pursuant to the Registration Statement (other than the Resale Shares); and b. the Ordinary Shares issuable pursuant to the conversion, exchange or exercise of other Registered Securities.

"Registration Statement"	The Company's registration statement on Form S-3 filed or to be filed with the SEC in the form reviewed by us.

"Relevant Moment"	Each time when one or more Resale Shares, Convertible Notes, Pre-Funded Warrants, Registered Shares or other Registered Securities are or were, as applicable, issued by the Company.

"Resale Shares"	The PIPE Shares, the Ordinary Shares underlying the Pre-Funded Warrants and the Ordinary Shares underlying the Convertible Notes (including the 3,818,704 Ordinary Shares issued pursuant to the signed private deed of issue of Ordinary Shares dated September 18, 2025), in each case being registered for resale pursuant to the Registration Statement.



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"Reviewed Documents"	The deeds, agreements and warrants explicitly referenced in the definitions of “Convertible Notes”, “PIPE Shares”, “Pre-Funded Warrants” and “Resale Shares”, in each case in the forms reviewed by us.

"SEC"	The United States Securities and Exchange Commission.

"Units"	One or more series of units issuable by the Company and registered pursuant to the Registration Statement consisting of one or more Debt Securities, Registered Shares, Warrants or any combination of such securities as specified in the applicable prospectus supplement.

"Warrants"	One or more series of warrants issuable by the Company and registered pursuant to the Registration Statement for the purchase of Debt Securities or Ordinary Shares as specified in the applicable prospectus supplement.