Gulf of ðzmit (), also referred to as ðzmit 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 ðzmit. Other cities and towns around the bay are Gebze, Körfez, Gölcük, and Altñnova.
In the eastâÂÂwest direction, it extends for a length of about , while in the northâÂÂsouth direction its width varies from at the narrowest spots to about at its widest. The ðzmit Bay Bridge is a suspension bridge that bridges the gulf.
The North Anatolian Fault Zone, the most prominent active fault in Turkey and the source of numerous large earthquakes throughout history, passes through the Gulf of ðzmit.
ðzmitâÂÂs ancient names were Gulf of Astacus, Sinus Astacenus (), Olbianus Sinus (), and Gulf of Nicomedia.
It took the names Gulf of Astacus and Sinus Astacenus from the city Astacus.