İzmit (Greek: Νικομήδεια, Nikomedeia) is a city in Turkey, the administrative center of the Kocaeli Province as well as the Metropolitan Municipality. It is located at the Gulf of İzmit (the ancient Gulf of Astacus) in the Sea of Marmara, about 100 km (62 mi) east of Istanbul, on the northwestern part of Anatolia. The city center has a population of 300,611 (2011 census).