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Preliminary Proxy Statement

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Definitive Proxy Statement

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Soliciting Material under §240.14a-12

MARINUS PHARMACEUTICALS, INC.

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*** Exercise Your Right to Vote *** Important Notice Regarding the Availability of Proxy Materials for the Stockholder Meeting to Be Held on May 7, 2019. MARINUS PHARMACEUTICALS, INC. XXXX XXXX XXXX XXXX (located on the following page). MARINUS PHARMACEUTICALS, INC. 170 N. RADNOR CHESTER RD, SUITE 250 RADNOR, PA 19087 You are receiving this communication because you hold shares in the company named above. This is not a ballot. You cannot use this notice to vote these shares. This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. You may view the proxy materials online at www.proxyvote.com or easily request a paper copy (see reverse side). We encourage you to access and review all of the important information contained in the proxy materials before voting. proxy materials and voting instructions. E67579-P19047 See the reverse side of this notice to obtain Meeting Information Meeting Type: Annual Meeting For holders as of: March 11, 2019 Date: May 7, 2019 Time: 10:00 AM Location: Meeting live via the Internet-please visit www.virtualshareholdermeeting.com/MRNS2019 The company will be hosting the meeting live via the Internet this year. To attend the meeting via the Internet please visit www.virtualshareholdermeeting.com/MRNS2019 and be sure to have the information that is printed in the box marked by the arrow

Before You Vote How to Access the Proxy Materials Have the information that is printed in the box marked by the arrow XXXX XXXX XXXX XXXX (located on the by the arrow XXXX XXXX XXXX XXXX (located on the following page) in the subject line. How To Vote Please Choose One of the Following Voting Methods XXXX XXXX XXXX XXXX (located on the following page) available and follow the instructions. the arrow XXXX XXXX XXXX XXXX (located on the following page) available and follow the instructions. E67580-P19047 Vote By Internet: Before The Meeting: Go to www.proxyvote.com. Have the information that is printed in the box marked by the arrow During The Meeting: Go to www.virtualshareholdermeeting.com/MRNS2019. Have the information that is printed in the box marked by Vote By Mail: You can vote by mail by requesting a paper copy of the materials, which will include a proxy card. Proxy Materials Available to VIEW or RECEIVE: NOTICE AND PROXY STATEMENTANNUAL REPORT How to View Online: following page) and visit: www.proxyvote.com. How to Request and Receive a PAPER or E-MAIL Copy: If you want to receive a paper or e-mail copy of these documents, you must request one. There is NO charge for requesting a copy. Please choose one of the following methods to make your request: 1) BY INTERNET:www.proxyvote.com 2) BY TELEPHONE:1-800-579-1639 3) BY E-MAIL*:sendmaterial@proxyvote.com * If requesting materials by e-mail, please send a blank e-mail with the information that is printed in the box marked Requests, instructions and other inquiries sent to this e-mail address will NOT be forwarded to your investment advisor. Please make the request as instructed above on or before April 23, 2019 to facilitate timely delivery.

THE BOARD OF DIRECTORS RECOMMENDS A VOTE "FOR" THE ELECTION OF DIRECTORS AND "FOR" PROPOSALS 2 AND 3. 1. The election as Class II directors of the nominees listed below. NOMINEES: 01) 02) 03) Scott Braunstein Seth H.Z. Fischer Nicole Vitullo 2. The approval of an amendment to our 2014 Plan to increase, on an annual basis, the maximum number of shares of common stock available for issuance under the 2014 Plan by a number of shares equal to the lesser of (i) 5,000,000 shares of common stock, (ii) an amount equal to 4% of the total number of shares of common stock outstanding on such date, or (iii) an amount determined by the Board, each of which may be granted as incentive stock options, and to increase the maximum aggregate number of shares of Company Stock that shall be subject to Grants to any individual during any calendar year to 2,000,000 shares of common stock. 3. The ratification of KPMG LLP as the independent registered public accounting firm for the fiscal year ending December 31, 2019. In their discretion, the proxies are authorized to vote upon such other business as may properly come before the Annual Meeting. E67581-P19047 Voting Items

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