Cius^{[pronunciation?]}, or Kios (Greek: Kίος), later renamed Prusias ad Mare after king Prusias I of Bithynia, was an ancient Greek city bordering the Propontis (now known as the Sea of Marmara), in Bithynia (in modern northwestern Turkey), and had a long history, being mentioned by Aristotle, Strabo and Apollonius Rhodius. It was colonized by the Milesians and became a place of much commercial importance. It joined the Aetolian League, and was destroyed by Philip V of Macedon in the Cretan War. It was rebuilt by Prusias I of Bithynia who renamed it for himself. An important chain in the ancient Silk Road, it became known as a wealthy town. Following the population exchange in 1923, the Greek refugees from Cius established the village of Nea Kios, in Argolis, Greece. There are only few remnants of the ancient town and its harbour today. Somewhat more to the west, the new modern town of Gemlik, Bursa Province, Turkey can be found.

Claudia Metrodora - 1st century public benefactor