Nicomedes I of Bithynia founded the city of Nicomedia (modern İzmit), at the head of the Gulf of Astacus (which opens on the Propontis), in 264 BC The city has ever since been one of the chief towns in this part of Asia Minor. It was the metropolis of Bithynia under the Roman empire (see Nicaea), and Diocletian made it the chief city of the Eastern Roman empire. Owing to its position at the convergence of the Asiatic roads to the new capital, Nicomedia retained its importance even after the foundation of Constantinople and its own capture by the Turks (1338).

See C. Texier, Asie mineure (Paris, 1839); V. Cuenet, Turquie d'Asie (Paris, 1894).