Byzantium, ancient city of Thrace, on the site of the present-day Istanbul, Turkey. Founded by Greeks from Megara in 667 B.C., it early rose to importance because of its position on the Bosporus. In the Peloponnesian War it was captured and recaptured by the contending forces. It was taken (A.D. 196) by Roman Emperor Septimius Severus. Constantine I ordered (A.D. 330) a new city built there; this was Constantinople, later the capital of the Byzantine Empire.