Exhibit 5.1

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September 16, 2021

Hercules Capital, Inc.

400 Hamilton Avenue, Suite 310

Palo Alto, CA 94301

Re: Registration Statement on Form N-2

Ladies and Gentlemen:

We have acted as counsel to Hercules Capital, Inc., a Maryland corporation (the “Company”), in connection with the preparation and filing of a Registration Statement on Form N-2 (File No. 333-231089), originally filed on April 29, 2019 with the U.S. Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933 (as amended, the “Securities Act”) and as subsequently amended on April 30, 2019 and May 6, 2019 (as amended, and including the documents incorporated by reference therein, the “Registration Statement”) and the final prospectus supplement, dated September 13, 2021 (including the base prospectus filed therewith and the documents incorporated by reference therein, the “Prospectus Supplement”), filed with the Commission on September 15, 2021 pursuant to Rule 424(b) under the Securities Act, relating to the proposed issuance by the Company of $325.0 million aggregate principal amount of 2.625% notes due 2026 (the “Notes”), to be sold to underwriters pursuant to an underwriting agreement substantially in the form filed with the Commission on September 15, 2021 on Form 8-K (the “Underwriting Agreement”). This opinion letter is being furnished to the Company in accordance with the requirements of Item 25 of Form N-2 under the Investment Company Act of 1940, as amended, and we express no opinion herein as to any matter other than as to the legality of the Indenture (as defined below) and the Notes.

The Notes are to be issued pursuant to the indenture dated as of March 6, 2012 (the “Base Indenture”), between the Company and U.S. Bank National Association, as trustee (the “Trustee”), as supplemented by the seventh supplemental indenture, dated as of September 16, 2021 (the “Seventh Supplemental Indenture” and together with the Base Indenture, the “Indenture”), between the Company and the Trustee.

In rendering the opinions expressed below, we have examined and relied on originals or copies, certified or otherwise identified to our satisfaction, of such documents, corporate records and other instruments and such agreements, certificates and receipts of public officials, certificates of officers or other representatives of the Company and others, and such other documents as we have deemed necessary or appropriate as a basis for the opinions set forth below, including the following documents:

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As to the facts upon which this opinion is based, we have relied, to the extent we deem proper, upon certificates of public officials and certificates and written statements of agents, officers, directors, employees and representatives of the Company without having independently verified such factual matters.

In our examination, we have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as original documents, the conformity to original documents of all documents submitted to us as copies, the legal capacity of natural persons who are signatories to the documents examined by us and the legal power and authority of all persons signing on behalf of the parties to such documents.

On the basis of the foregoing and subject to the assumptions, qualifications and limitations set forth in this letter, we are of the opinion that:

The opinions set forth herein are subject to the following assumptions, qualifications, limitations and exceptions being true and correct at or before the issuance of the Notes:

The opinions set forth herein as to enforceability of obligations of the Company are subject to: (i) bankruptcy, insolvency, reorganization, fraudulent conveyance, moratorium or similar laws now or hereinafter in effect affecting the enforcement of creditors’ rights generally, and by general principles of equity (regardless of whether enforcement is sought in a proceeding in equity or at law) and the discretion of the court or other body before which any proceeding may be brought; (ii) the unenforceability under certain circumstances under law or court decisions of

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provisions providing for the indemnification of, or contribution to, a party with respect to a liability where such indemnification or contribution is contrary to public policy; (iii) provisions of law which may require that a judgment for money damages rendered by a court in the United States be expressed only in U.S. dollars; (iv) requirements that a claim with respect to any debt securities denominated other than in U.S. dollars (or a judgment denominated other than in U.S. dollars in respect of such claim) be converted into U.S. dollars at a rate of exchange prevailing on a date determined pursuant to applicable law; and (v) governmental authority to limit, delay or prohibit the making of payments outside the United States or in foreign currency or composite currency.

We express no opinion as to the validity, legally binding effect or enforceability of any provision in any agreement or instrument that (i) requires or relates to payment of any interest at a rate or in an amount which a court may determine in the circumstances under applicable law to be commercially unreasonable or a penalty or forfeiture or (ii) relates to governing law and submission by the parties to the jurisdiction of one or more particular courts.

The opinions expressed herein are limited to the federal laws of the United States of America and the laws of the State of New York. We are members of the bar of the State of New York.

We assume no obligation to advise you of any changes in the foregoing subsequent to the date of this opinion.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and to the reference to this firm under the caption “Legal Matters” in the prospectus that forms a part of the Registration Statement and the Prospectus Supplement. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission thereunder.

Very truly yours,

/s/ Dechert LLP