Exhibit 5.1

Hogan Lovells US LLP
1735 Market Street, Floor 23
Philadelphia, PA 19103
T +1 267 675 4600
F +1 267 675 4601
www.hoganlovells.com

November 10, 2022

Board of Directors

Marinus Pharmaceuticals, Inc.

5 Radnor Corporate Center, Suite 500

100 Matsonford Road

Radnor, PA 19087

Ladies and Gentlemen:

We are acting as counsel to Marinus Pharmaceuticals, Inc., a Delaware corporation (the “Company”), in connection with its registration statement on Form S-3, as amended (File No. 333-239780) (the “Registration Statement”), filed with the Securities and Exchange Commission under the Securities Act of 1933, as amended (the “Act”), relating to the public offering of up to (i) 12,421,053 shares of common stock, par value $0.001 per share (“Common Stock”) of the Company (the “Shares”), and (ii) pre-funded warrants (the “Warrants” and, together with the Shares, the “Securities”) to purchase an aggregate of 2,105,264 shares of Common Stock with an exercise price equal to $0.001 per share (the “Warrant Shares”), all of which Securities are to be sold by the Company pursuant to the Underwriting Agreement, dated as of November 7, 2022, among the Company and the underwriters named therein (the “Underwriting Agreement”), as described in the prospectus, dated July 27, 2020 (the “Registration Statement Prospectus”), which forms a part of the Registration Statement, as supplemented by the Prospectus Supplement, dated November 7, 2022 (together with the Registration Statement Prospectus, the “Prospectus”). This opinion letter is furnished to you at your request to enable you to fulfill the requirements of Item 601(b)(5) of Regulation S-K, 17 C.F.R. § 229.601(b)(5), in connection with the Registration Statement.

For purposes of this opinion letter, we have examined copies of such agreements, instruments and documents as we have deemed an appropriate basis on which to render the opinions hereinafter expressed. In our examination of the aforesaid documents, we have assumed the genuineness of all signatures, the legal capacity of all natural persons, the accuracy and completeness of all documents submitted to us, the authenticity of all original documents, and the conformity to authentic original documents of all documents submitted to us as copies (including pdfs). As to all matters of fact, we have relied on the representations and statements of fact made in the documents so reviewed, and we have not independently established the facts so relied on. This opinion letter is given, and all statements herein are made, in the context of the foregoing.

This opinion letter is based as to matters of law solely on the Delaware General Corporation Law, as amended. We express no opinion herein as to any other statutes, rules or regulations.

Hogan Lovells US LLP is a limited liability partnership registered in the District of Columbia.  “Hogan Lovells” is an international legal practice that includes Hogan Lovells US LLP and Hogan Lovells International LLP, with offices in:  Alicante  Amsterdam  Baltimore  Beijing  Birmingham  Boston  Brussels  Colorado Springs  Denver  Dubai  Dusseldorf  Frankfurt  Hamburg  Hanoi  Ho Chi Minh City  Hong Kong  Houston  Johannesburg  London  Los Angeles  Luxembourg  Madrid  Mexico City  Miami  Milan  Minneapolis  Monterrey  Munich  New York  Northern Virginia  Paris  Perth  Philadelphia  Rome  San Francisco  São Paulo  Shanghai  Silicon Valley  Singapore  Sydney  Tokyo  Warsaw  Washington, D.C.   Associated Offices:  Budapest  Jakarta  Riyadh  Shanghai FTZ  Ulaanbaatar.   Business Service Centers:  Johannesburg  Louisville.   Legal Services Center:  Berlin.  For more information see www.hoganlovells.com

Marinus Pharmaceuticals, Inc. - 2 - November 10, 2022

Based upon, subject to and limited by the foregoing, we are of the opinion that:

1. Following (i) issuance of the Shares pursuant to the terms of the Underwriting Agreement and (ii) receipt by the Company of the consideration for the Shares specified in the resolutions of the Board of Directors and the Pricing Committee of the Board of Directors, the Shares will be validly issued, fully paid, and nonassessable.

2. Following (i) execution and delivery by the Company of the Warrants pursuant to the terms of the Underwriting Agreement and (ii) receipt by the Company of the consideration for the Warrants specified in the resolutions of the Board of Directors and the Pricing Committee of the Board of Directors, the Warrants will constitute valid and binding obligations of the Company.

3. The Warrant Shares have been duly authorized and reserved for issuance and, when issued, delivered and paid for upon exercise in accordance with the provisions of the Warrants, will be validly issued, fully paid, and nonassessable.

This opinion letter has been prepared for use in connection with the Registration Statement. We assume no obligation to advise of any changes in the foregoing subsequent to the effective date of the Registration Statement.

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Company’s Current Report on Form 8-K to be filed with the Securities and Exchange Commission on the date hereof and to the reference to this firm under the caption “Legal Matters” in the Prospectus constituting a part of the Registration Statement. In giving this consent, we do not thereby admit that we are an “expert” within the meaning of the Act.

Very truly yours,

/s/ HOGAN LOVELLS US LLP

HOGAN LOVELLS US LLP