Gebze (Turkish pronunciation: [ˈɟebze],) is a district in Kocaeli Province, Turkey. It is situated 65 km (30 mi) southeast of Istanbul, on the Gulf of Izmit, the eastern arm of the Sea of Marmara. Gebze is the second largest district in Kocaeli after İzmit, the provincial centrum in terms of population. Gebze has experienced rapid growth in recent years, from 159,116 residents in 1990 to 299,047 in 2011.
The district of Gebze is located in the westernmost portion of Kocaeli Province, with neighbors Tuzla to the west; Pendik, northwest; Şile, north; and Körfez, to the east. The district includes 6 municipalities (Gebze, Darıca, Çayırova, Dilovası, Şekerpınar, and Tavşancıl) and 22 villages (Ahatlı, Balçık, Cumaköy, Çerkeşli, Demirciler, Denizli, Duraklı, Elbizli, Eskihisar, Hatipler, Kadıllı, Kargalı, Köseler, Mollafenari, Muallimköy, Mudarlı, Ovacık, Pelitli, Tavşanlı, Tepecik, Tepemanayır (Tepepanayır), and Yağcılar).
The northern terminus of Osman Gazi Bridge falls within this area; the construction — having a total length of 4 kilometers (with a 1,688-meter main span) — bridges the Sea of Marmara from Kababurun to Dilburnu. The Gebze Metro began construction in 2018 for a 2023 opening.