Izmit, known as Nicomedia in antiquity, is a district and the capital of Kocaeli province, Turkey. It is located at the Gulf of Izmit 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). The population of the province (including rural areas) is 1,459,772. Unlike other provinces in Turkey, apart from Istanbul, the whole province is included within the municipality of the metropolitan center.

Nicomedia was the eastern and most senior capital city of the Roman Empire between 286 and 324, during the Tetrarchy introduced by Diocletian. Following Constantine the Great's victory over co-emperor Licinius at the Battle of Chrysopolis in 324, Nicomedia served as an interim capital city for Nova Roma.

The city is mostly built on hill slopes because of the cramped area, while flat plains surround the gulf, near the sea. This topographic structure divided the city into two parts. The first was created on flat plains, where the city center is. The railway and highway networks pass from this area which is close to the Sea of Marmara. The second part was built on hills, with many historic houses from the Ottoman period in the old quarters.