Nicomedia (nĬkōmē´dēə), ancient city, NW Asia Minor, near the Bosporus, in present-day Turkey. Refounded (264 BC) by Nicomedes I of Bithynia to replace Astacus as his capital, it flourished for centuries. The Goths sacked the city in AD 258. Diocletian chose it for the eastern imperial capital, but it was soon superseded by Byzantium (Constantinople). The modern city on its site is Izmit.