UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 1)*

DelcathSystems, Inc.

(Name of Issuer)

Common Stock, par value $0.01 per share

(Title of Class of Securities)

24661P807

(CUSIP Number)

McCarter & English, LLP

Four Gateway Center

100Mulberry Street

Newark, New Jersey 07102

Attention: Veronica H. Montagna

973-639-7948

(Name,Address and Telephone Number of Person Authorized to Receive Notices and Communications)

December 13, 2022

(Date of Event which Requires Filing of this Statement)

If the filingperson has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the followingbox.  ☐

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of theSecurities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

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Explanatory Note

This statement constitutes Amendment No. 1 (“Amendment No. 1”) to the Statement on Schedule 13D filed on July 27,2022 (the “Original Schedule 13D”) by Gerard Michel (the “Reporting Person”) relating to his beneficial ownership of the common stock, par value $0.01 per share (the “Common Stock”), of Delcath Systems, Inc., a Delawarecorporation (the “Company”).

In accordance with Rule 13d-2 of the Securities ExchangeAct of 1934, as amended, this Amendment No. 1 amends and supplements only information that has materially changed since the filing of the Original Schedule 13D, including the disclosure of the number of shares of the Company’s Common Stockbeneficially owned or deemed to be beneficially owned by the Reporting Person. Unless otherwise stated, the information set forth in the Original Schedule 13D remains accurate in all material respects.

Item 4. Purpose of Transaction.

Item 4 is hereby amended and supplemented by adding the following:

On December 13, 2022, the Company and certain accredited investors, including the Reporting Person, entered into a securities purchaseagreement (the “Securities Purchase Agreement”) pursuant to which the Company agreed to sell and issue to the investors in a private placement (the “Private Placement”): (i) an aggregate of 1,448,889 shares of Common Stock at apurchase price of $2.90 per share and (ii) in lieu of shares of Common Stock, 692,042 pre-funded warrants to purchase Common Stock at a purchase price of $2.89 perpre-funded warrant, which pre-funded warrants have an exercise price of $0.01 per share of Common Stock and are immediately exercisable and remain exercisable untilexercised in full. The Reporting Person purchased and was issued 51,725 shares of Common Stock in the Private Placement, which closed on December 13, 2022.

In connection with the Private Placement, the Company and the investors entered into a registration rights agreement dated December 7,2022 (the “Registration Rights Agreement”), providing for the registration for resale of the securities (including the shares of Common Stock underlying the pre-funded warrants) issued under theSecurities Purchase Agreement that are not then registered on an effective registration statement, pursuant to a registration statement to be filed with the Securities and Exchange Commission on or prior to February 6, 2023.

The foregoing descriptions of the Securities Purchase Agreement and the Registration Rights Agreement do not purport to be complete and arequalified in their entirety by reference to the Securities Purchase Agreement and Registration Rights Agreement, copies of which are filed as Exhibits A and B hereto and incorporated herein by reference.

The Reporting Person holds the Common Stock for investment purposes. The Securities Purchase Agreement and the Registration Rights Agreementwere not entered into, and the shares of Common Stock were not acquired by the Reporting Person, and are not held by the Reporting Person for the purpose or with the effect of changing or influencing the control of the Company. The Reporting Personmay, from time to time, acquire additional shares of Common Stock and/or retain and/or sell all or a portion of the shares of Common Stock held by the Reporting Person in open market or in privately negotiated transactions, and/or may distribute the

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Common Stock held by the Reporting Person to other persons. All transactions involving Common Stock by the Reporting Person are subject to the Company’s policies, including its policies withrespect to insider trading. Except as set forth above, the Reporting Person has no present plans or intentions which would result in or relate to any of the transactions described in subparagraphs (a) through (j) of Item 4 of Schedule 13D.

Item 5. Interest in Securities of the Issuer.

Item 6. Contracts, Arrangements, Understandings or Relationships with Respect to Securities of the Issuer.

The information set forth, or incorporated by reference, in Items 4 and 5 above is hereby incorporated by this reference in this Item6. To the Reporting Person’s knowledge, except as otherwise described in Item 4 hereof and in Item 4 of the Original Schedule 13D, there are no contracts, arrangements, understandings or relationships (legal or otherwise) between theReporting Person and any person with respect to any securities of the Company, including but not limited to transfer or voting of any of the securities, finder’s fees, joint ventures, loan or option arrangements, puts or calls, guarantees ofprofits, division of profits or loss, or the giving or withholding of proxies.

Item 7. Material to Be Filed as Exhibits.

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SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true,complete and correct.

Dated: December 16, 2022

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