İzmit (Greek: Νικομήδεια, Nicomedia) is a city in Turkey, administrative center of Kocaeli Province as well as the Kocaeli 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 293,339 (2010 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 now 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's victory over co-emperor Licinius at the Battle of Chrysopolis (Üsküdar) in 324, Nicomedia served as an interim capital city for Constantine the Great between 324 and 330, until the nearby Byzantium was officially declared Nova Roma (later known as Constantinople, present-day Istanbul).