ancient Greek town, located on the southern coast of the Sea of Marmara in what is now Balikhisar, Tur. It was probably founded as a colony of Miletus in 756 BC, and its advantageous position soon gave it commercial importance.

With the collapse of Lydia (c. 544), Cyzicus came under Persian rule, but it joined the Delian League after the defeat of Xerxes and remained loyal to Athens until 411 BC. In Hellenistic times the town was at first closely connected with Pergamum and later allied itself with Rome. Cyzicus was partly destroyed by the Arabs in AD 675 and probably further damaged by a series of earthquakes. Among its ruins are the substructures of the Temple of Hadrian, sometimes ranked among the Seven Wonders of the World.