UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 6-K

REPORT OF FOREIGN PRIVATE ISSUER

PURSUANT TO RULE 13a-16 OR 15d-16

UNDER THE SECURITIES EXCHANGE ACT OF 1934

For the month of September 2026

Commission File Number: 001-15092

TURKCELL İLETİŞİM HİZMETLERİ A.Ş.

(Translation of registrant’s name into English)

Turkcell Küçükyalı Plaza

Aydınevler Mahallesi, İsmet İnönü Caddesi, No:20

Küçükyalı B Blok Ofispark

34854 Maltepe
Istanbul, Türkiye

(Address of Principal Executive Offices)

Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F:

x Form 20-F  ¨ Form 40-F

Enclosure: A press release dated September 24, 2026 announcing the registrant’s decision to increase the capital of BeST.

Istanbul, September 24, 2026

Announcement Regarding the Capital Increase in BeST

Our Company’s Board of Directors has resolved to increase the capital of our subsidiary CJSC Belarusian Telecommunications Network (“BeST”) by a total amount of USD 7,000,000 and/or the equivalent amount in foreign currency, with the first installment of USD 5,000,000 and/or the equivalent amount in foreign currency to be paid in October 2026 and the second installment of USD 2,000,000 and/or the equivalent amount in foreign currency to be paid in December 2026.

For more information:

Turkcell Investor Relations

investor.relations@turkcell.com.tr

Tel: + 90 212 313 1888

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, Turkcell İletişim Hizmetleri A.Ş. has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.