Gulf of İzmit (Turkish: İzmit körfezi), also referred to as İzmit Bay, is a bay at the easternmost edge of the Sea of Marmara, in Kocaeli Province, Turkey. The gulf takes its name from the city of İzmit. Other cities and towns around the bay are Gebze, Körfez, Gölcük, and Altınova.
In the east–west direction, it extends for a length of about 48 kilometres (30 mi), while in the north–south direction its width varies from 2 to 3 kilometres (1.2 to 1.9 mi) at the narrowest spots to about 10 kilometres (6.2 mi) at its widest. The İzmit Bay Bridge is a suspension bridge that bridges the gulf.