Exhibit 5.1

Hogan Lovells US LLP

Harbor East

100 International Drive

Suite 2000

Baltimore, MD 21202

T +1 410 659 2700

F +1 410 659 2701

www.hoganlovells.com

May 13, 2019

Board of Directors

NextCure, Inc.

9000 Virginia Manor Road

Suite 200

Beltsville, MD 21202

Ladies and Gentlemen:

We are acting as counsel to NextCure, Inc., a Delaware corporation (the “Company”), in connection with its registration statement on Form S-8 (the “Registration Statement”), filed with the Securities and Exchange Commission under the Securities Act of 1933, as amended (the “Act”) relating to the proposed offering of up to 240,000 newly issued shares of the common stock, par value $0.001 per share (the “Common Stock”) of the Company (the “Shares”), all of which are available for issuance under the NextCure, Inc. 2019 Employee Stock Purchase Plan (the “ESPP”). 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.

Based upon, subject to and limited by the foregoing, we are of the opinion that following (i) effectiveness of the Registration Statement, (ii) issuance of the Shares pursuant to the terms of the ESPP, and (iii) receipt by the Company of the consideration for the Shares specified in the applicable resolutions of the Board of Directors and in the ESPP, the Shares will be validly issued, fully paid, and nonassessable.

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 Moscow Munich New York Northern Virginia Paris Perth Philadelphia Rio de Janeiro Rome San Francisco São Paulo Shanghai Silicon Valley Singapore Sydney Tokyo Warsaw Washington DC Associated offices: Budapest Jakarta Shanghai FTZ Ulaanbaatar Zagreb. Business Service Centers: Johannesburg Louisville. For more information see www.hoganlovells.com

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 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