Gulf of İzmit (Turkish: İzmit Körfezi), anciently, Sinus Astacenus and Olbianus Sinus (Ancient Greek: Ὀλβιανὸς κόλπος, romanized: Olbianos kolpos), also referred to as Izmit Bay, is a bay at the easternmost edge of the Sea of Marmara, in Kocaeli Province, Turkey.^[1] In antiquity it was known first as the Gulf of Astacus and later as the Gulf of Nicomedia.^[2]

The gulf takes its name from the city of İzmit, other cities and towns around the bay being 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.