UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

(RULE 14a-101)

INFORMATION REQUIRED IN PROXY STATEMENT

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the Securities

Exchange Act of 1934

Filed by the Registrant ☒

Filed by a Party other than the Registrant ☐

Check the appropriate box:

☐ Preliminary Proxy Statement

☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

☐ Definitive Proxy Statement

☒ Definitive Additional Materials

☐ Soliciting Material under §240.14a-12

Runway Growth Finance Corp.

(Name of Registrant as Specified in Its Charter)

(Name of Person(s) Filing Proxy Statement if Other Than the Registrant)

☒ No fee required.

☐ Fee paid previously with preliminary materials.

☐ Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-11

Important Notice Regarding the Availability of Proxy Materials for theAnnual Meeting of Stockholders of RUNWAY GROWTH FINANCE CORP.To Be Held Virtually On:June 23, 2026, at 9:00 am Central Time COMPANY NUMBERACCOUNT NUMBERCONTROL NUMBER This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. Weencourage you to access and review all of the important information contained in the proxy materials before voting.If you want to receive a paper or e-mail copy of the proxy materials you must request one. There is no charge to you for requesting a copy.To facilitate timely delivery please make the request as instructed below before June 9, 2026.Please visit www.astproxyportal.com/ast/22600, where the following materials are available for view:• Notice of Meeting• Proxy Statement• Proxy Card• Form 10-K• Company LetterTO REQUEST MATERIAL: TELEPHONE: 1-888-Proxy-NA (1-888-776-9962) or +1-201-299-6210 (worldwide)E-MAIL: help@equiniti.comWEBSITE: us.astfinancial.com/OnlineProxyVoting/ProxyVoting/RequestMaterialsTO VOTE: ONLINE: To access your online proxy card, please visit www.voteproxy.com and follow the on-screeninstructions or scan the QR code with your smartphone. You may enter your voting instructions atwww.voteproxy.com up until 11:59 pm Eastern Time the day before the meeting.VIRTUALLY AT THE MEETING: The Company will be hosting the meeting virtually this year. To attend thevirtual meeting, please visit edge.media-server.com/mmc/p/grqssx8a and be sure to have your controlnumber available. The meeting password is runway2026.MAIL: You may request a card by following the instructions above. 1. To elect two Class I directors who will each serve for a term of three yearsand until their successors are elected and qualified. NOMINEES: Alexander DukaGary Kovacs 2. To ratify the selection of Deloitte & Touche LLP to serve as the Company’s independentregistered public accounting firm for the fiscal year ending December 31, 2026.THE BOARD OF DIRECTORS RECOMMENDS A VOTE "FOR ALL NOMINEES" IN THEELECTION OF DIRECTORS AND "FOR" PROPOSAL 2. Please note that you cannot use this notice to vote by mail.