Gulf of ?zmit (Turkish: ?zmit Körfezi), also referred to as Izmit Bay, is a bay at the easternmost edge of the Sea of Marmara, in Kocaeli Province, Turkey. Its ancient names were Gulf of Astacus, Sinus Astacenus (Ancient Greek: ), Olbianus Sinus (Ancient Greek: , romanized: Olbianos kolpos) and Gulf of Nicomedia.
The gulf takes its name from the city of ?zmit, other cities and towns around the bay being Gebze, Körfez, Gölcük, and Alt?nova. 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 ?zmit Bay Bridge is a suspension bridge that bridges the gulf.