GCT Semiconductor Holding Inc美股招股说明书(2025-12-15版)

$20,000,000 Principal Amount of Convertible Promissory Notes and Common Stock Underlying Convertible Promissory Notes We are offering by this prospectus supplement (i)up to $20,000,000 aggregate principal amount of a series of unsecured, interest-freemandatory convertible promissory notes (the “Convertible Notes”) to Indigo Capital LP (the “Investor”), and (ii)the shares ofcommon stock issuable from time to time upon conversion of the Convertible Notes. On December15, 2025, we entered into a Convertible Promissory Note Purchase Agreement with the Investor (the “PurchaseAgreement”) providing for the potential purchase by the Investor of up to $20,000,000 aggregate principal amount of ConvertibleNotes. The Convertible Notes, when issued, will constitute unsecured obligations of the Company and will rank pari passu with all ofour other unsecured indebtedness. The Convertible Notes will be issued in tranches as described below, at a subscription price equal to93.00% of the par value of each tranche, will bear no interest, and will mature 24 months from the date of issuance unless earlierconverted or redeemed in accordance with their terms. For a more detailed description of the Purchase Agreement and the ConvertibleNotes, please see our Current Report on Form8-K filed with the SEC on December15, 2025, which we incorporate herein byreference. We may draw upon the initial tranche of $1,000,000 upon the filing of this prospectus supplement under Rule424(b)(5)and thedelivery of an issuer’s counsel opinion in connection with our effective shelf registration statement on FormS-3 (File No.333-286316), originally filed on April1, 2025 and declared effective on April9, 2025. Following the initial tranche, we have the right, butnot the obligation, to submit weekly purchase notices to the Investor for additional tranches of up to $1,000,000 each, provided thatour common stock has maintained a closing bid price of at least $1.00 for five consecutive trading days prior to the applicable noticedate. Funding of any tranche during a period in which our share price is below $1.00 will require mutual agreement between us and theInvestor. The Investor may convert the funded principal of each Convertible Note at any time prior to maturity at a conversion price equal to90.00% of the lowest volume-weighted average price (“VWAP”) of our common stock during the three trading days prior to the dateon which the Investor delivers a notice of conversion. Conversion of each Convertible Note will be mandatory at its maturity unlesswe elect to repay all outstanding amounts in cash at a premium of 114% of such amount. We may also redeem any Convertible Note incash at 107% of par value after the first anniversary of issuance or at 114% of par value after 18 months from issuance. We have agreed to maintain, at all times while any Convertible Notes are outstanding, a reserve of authorized but unissued shares ofcommon stock equal to at least two times the number of shares issuable upon full conversion of all outstanding Convertible Notes. The registration of the issuance of common stock hereunder does not necessarily mean that the Investor will convert any ConvertibleNotes. We will not receive any proceeds from the issuance of shares upon conversion of the Convertible Notes, but we will receiveproceeds from the sale of Convertible Notes issued to the Investor pursuant to the Purchase Agreement. We currently intend to use thenet proceeds from this offering for working capital and general corporate purposes, including repayment of certain outstandingindebtedness. We have agreed to pay certain expenses relating to the registration of such shares with the U.S. Securities and ExchangeCommission. See “Plan of Distribution” in this prospectus supplement. No public market currently exists for the Convertible Notes, and we do not intend to apply to list the Convertible Notes on anysecurities exchange or for quotation on any inter-dealer quotation system. Our common stock is listed on The New York StockExchange (the “NYSE”), under the symbol “GCTS.” On December12, 2025, the closing price of our common stock on the NYSE was$1.45 per share. We are an “emerging growth company” under federal securities laws and are subject to reduced public company reportingrequirements. Investing in our securities involves risks that are described in the “Risk Factors” section beginning on pageS-5of this prospectus supplement and any risk factors that are included in our filings with the Securities and ExchangeCommission, or the SEC, that are incorporated by reference herein. Neither the SEC nor any state securities commission has approved or disapproved of these securities or determined if this prospectussupplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense. The date of this prospectus supplement is December15, 2025. TABLE OF CONTENTS Prospectus Supplement ABOUT THIS PROSPECTUS SUPPLEMENTS-1