Cius (/ˈsaɪəs/; Ancient Greek: Kίος or Κῖος Kios), later renamed Prusias on the Sea (/ˈpruːʒəs/; Latin: 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 and in Mysia (in modern northwestern Turkey), and had a long history, being mentioned by Herodotus, Xenophon, Aristotle, Strabo and Apollonius Rhodius.

[3] It was at the foot of Mount Arganthonius, and there was a myth that Hylas, one of the companions of Heracles on the voyage to Colchis, was carried off by the nymphs when he went to get water here; and also that Cius, another companion of Heracles, on his return from Colchis, stayed here and founded the city, to which he gave his name.

The position of Cius made it the port for the inland parts, and it became a place of much commercial importance.

[1] It joined the Aetolian League, and was destroyed by Philip V of Macedon in the Second Macedonian War (200-197), and given by him to Prusias I of Bithynia.

[6][7] It was sometimes called Prusias ἐπιθαλασσίη, or "on the sea," to distinguish it from other towns of the same name,[8][9] or πρὸς θάλασσαν.

At first a suffragan of Nicomedia, it soon became an autocephalous archdiocese, being listed as such in Notitiae Episcopatuum from the 7th century onward.