İzmit (Turkish pronunciation: [ˈizmit]) is a municipality and the capital district of Kocaeli Province, Turkey. Its area is 480 km², and its population is 376,056 (2022). The capital of Kocaeli Province, it is located at the Gulf of İzmit in the Sea of Marmara, about 100 km (62 mi) east of Istanbul, on the northwestern part of Anatolia. Kocaeli Province (including rural areas) had a population of 2,079,072 inhabitants in 2022, of whom approximately 1.2 million lived in the largely urban İzmit City metro area made up of Kartepe, Başiskele, Körfez, Gölcük, Derince and Sapanca (in Sakarya Province). Similar to Istanbul, the area of İzmit is coterminous with its province. It is also the most populated of any city or town in Turkey whose name isn't shared with the province it is located in.

İzmit was known as Nicomedia and Ólbia in antiquity, and 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 Constantine between 324 and 330. During the Ottoman Empire, İzmit was the capital of the Sanjak of Kocaeli. In the present day, Istanbul-İzmit area is one of the main industrial regions in Turkey.

İzmit was known as Nicomedia (Ancient Greek: Νικομήδεια) and Ólbia (Ancient Greek: Ὀλβία) in antiquity. İzmit derives from Nicomedia, prefixed with εἰς 'to' or 'into' (similarly to İstanbul). Names used in English prior to official Turkish Latinization include Ismid, Iskimid, and Isnikmid.

The geographical location of İzmit is between 40°-41° N and 29°-31° E, surrounded by the Gulf of İzmit at south, Istanbul and the Sea of Marmara at west, the Black Sea at north, and Sakarya at east.

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 located. 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.

In Antiquity, the city in Greek was called Astacus or Olbia (founded 712 BC). After being destroyed, it was rebuilt and founded by Nicomedes I of Bithynia in 264 BC under the name of Nicomedia. It remained one of the most important cities in northwestern Asia Minor.

Carthaginian general and statesman Hannibal came to Nicomedia in his final years and committed suicide in nearby Libyssa (modern Gebze), in a date between 183 and 181 BC.

The historian Arrian was born in Nicomedia, which was the metropolis of Bithynia under the Roman Empire (see also Nicaea).