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 F +31 20 71 71 111	Amsterdam, March 16, 2021. To the Company

	Ladies and Gentlemen,
	We have acted as legal counsel as to Dutch law to the Company in connection with the Registration Statement and the filing thereof 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 addressed solely to you. 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.
	In rendering the opinions expressed in this opinion letter, we have exclusively reviewed and relied upon a draft of the Registration Statement, the Plan and pdf copies of the Corporate Documents and we have assumed that any grant of Awards and any issuance of Plan Shares 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.
Amsterdam Brussels	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, tax law, data protection law, securitization 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 Plan or the Corporate Documents subsequent to the date of this opinion letter.
London Luxemburg New York Rotterdam	This communication is confidential and may be subject to professional privilege. 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.

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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. 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.   drafts of documents reviewed by us will be signed in the form of those drafts, 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 Deed of Incorporation and the Deed of Conversion are valid notarial deeds and the Deed of Incorporation has been executed on the basis of a valid declaration of no objection (verklaring van geen bezwaar);

d.  at each Relevant Moment, the Current Articles are the Articles of Association in force and effect;

e.   at each Relevant Moment, the Company will not have (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, (iv) had its assets placed under administration (onder bewind gesteld), (v) been declared bankrupt (failliet verklaard), (vi) been

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granted a suspension of payments (surseance van betaling verleend), (vii) become subject to statutory proceedings for the restructuring of its debts (akkoordprocedure) (and no such proceedings will have been started) or (vi) have been made subject to similar proceedings in any jurisdiction or otherwise been limited in its power to dispose of its assets;

f.   at each Relevant Moment, (i) the relevant Award(s) shall have been validly granted as a right to subscribe for Ordinary Shares (recht tot het nemen van aandelen) by the corporate body authorized to do so, (ii) shall be in full force and effect upon being exercised or settled, as applicable, (iii) shall have been validly exercised or settled, as applicable, in accordance with the terms and conditions applicable to such Award(s) and (iv) any pre-emption rights in respect of such Award(s) shall have been validly excluded by the corporate body authorized to do so;

g.  any offering of Awards, to the extent made in the Netherlands, has been, is and will be made in conformity with the Prospectus Regulation and the rules promulgated thereunder;

h.  at each Relevant Moment, each holder of the relevant Award(s) shall be an individual who has not (i) deceased, (ii) had his/her assets placed under administration (onder bewind gesteld), (iii) been declared bankrupt (failliet verklaard), (iv) been granted a suspension of payments (surseance van betaling verleend), (v) subject to statutory proceedings for the restructuring of its debts (akkoordprocedure) (and no such proceedings will have been started wih respect to such individual) or (vi) been made subject to similar proceedings in any jurisdiction or otherwise been limited in the power to dispose of his/her assets; and

i.   at each Relevant Moment, the authorised share capital (maatschappelijk kapitaal) of the Company shall allow for the grant of Awards and the issuance of Plan Shares pursuant to the exercise or settlement thereof.

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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Plan Shares

2.  Subject to receipt by the Company of payment in full for the Plan Shares in accordance with the Plan, and when issued and accepted in accordance with the Plan, the Plan Shares will be validly issued, fully paid and non-assessable.

The opinions expressed above are subject to the following qualifications:

A. 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.

B. 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 clauses contained in the Current Articles, we have no reason to believe that, by issuing Plan Shares, the Company will transgress the description of the objects contained in the Current Articles. 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 Plan Shares since this is a matter of fact.

C. In the event any of the documents reviewed by us are signed by way of an electronic signature, such signature will have the same legal effect as a handwritten (“wet ink”) signature only if (i) it is a qualified electronic signature within the meaning of the eIDAS Regulation or (ii) the method used for signing is sufficiently reliable, taking the purpose for which the electronic signature was used and all other circumstances at hand into account. There is only limited case law and other legal guidance available

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to determine whether a method of electronic signing other than by way of a qualified electronic signature is “sufficiently reliable”. The legislature indicated that the parties to a document have a certain discretion in determining what method is “sufficiently reliable” for signing such document. In any case, any electronic signature or other electronic record purporting to have similar effect will be admissible into evidence in accordance with the DCCP.

D. Pursuant to Section 2:98c DCC, a naamloze vennootschap 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). Based on the Plan, we have no reason to believe that the Company or its subsidiaries will violate Section 2:98c DCC in connection with the issue of Plan Shares. However, we cannot confirm this definitively, since the determination of whether a company (or a subsidiary) has provided security, has given a price guarantee or has otherwise bound itself, with a view to the subscription or acquisition by third parties of shares in its share capital or depository receipts, as described above, is a matter of fact.

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

a.   rules relating to Insolvency Proceedings or similar proceedings under a foreign law and other rules affecting creditors’ rights generally, including the Temporary Covid-19 Act;

b.  the provisions of fraudulent preference and fraudulent conveyance (Actio Pauliana) and similar rights available in other jurisdictions to liquidators in bankruptcy proceedings or creditors;

c.   claims based on tort (onrechtmatige daad);

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

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

f.   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

g.  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).

F.  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 a share will 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 share.

G. 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.

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.
“Awards”	Rights to subscribe for Common Shares pursuant to the terms and conditions of the Plan.
“Bankruptcy Code”	The Dutch Bankrupcty Code (Faillissementswet).
“Commercial Register”	The Dutch Commercial Register (handelsregister).
“Common Shares”	Common shares in the Company’s capital, with a nominal value of EUR 0.09 each.
“Company”	Merus N.V., a public company with limited liability (naamloze vennootschap), registered with the Commercial Register under number 30189136.
“Corporate Documents”	The Deed of Incorporation, the Deed of Conversion, the Deed of Amendment and the Current Articles.

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“Current Articles”	The Articles of Association as they read immediately after the execution of the Deed of Amendment.
“DCC”	The Dutch Civil Code (Burgerlijk Wetboek).
“DCCP”	The Dutch Code on Civil Procedures (Wetboek van Burgerlijke Rechtsvordering).
“Deed of Amendment”	The deed of amendment to the Articles of Association, dated December 6, 2019.
“Deed of Conversion”	The deed of conversion and amendment to the Articles of Association, dated May 19, 2016.
“Deed of Incorporation”	The Company’s deed of incorporation (akte van oprichting), dated June 16, 2003.
“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 listed in Annex A thereto and any statutory proceedings for the restructuring of debts (akkoordprocedure) pursuant to the Bankruptcy Code.
“NautaDutilh”	NautaDutilh N.V.
“the Netherlands”	The European territory of the Kingdom of the Netherlands.

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“Plan”	The Merus N.V. 2016 Incentive Award Plan attached as an exhibit to the Registration Statement.
“Plan Shares”	Up to 2,419,406 Common Shares available for issuance under the Plan pursuant to the exercise of Awards.
“Registration Statement”	The Company’s registration statement on Form S-8 filed or to be filed with the SEC on or about the date of this opinion letter.
“Relevant Moment”	Each time when one or more Awards are issued or one or more Plan Shares are issued pursuant to the exercise or settlement of the relevant Award(s).
“SEC”	The United States Securities and Exchange Commission.
“Temporary Covid-19 Act”	Tijdelijke Wet COVID-19 SZW en JenV.