Cius (; Kios), later renamed Prusias on the Sea (; ) 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 Second Macedonian War (200-197 BC). 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.
Cius became an early Christian bishopric. Its bishop, Cyrillus, took part in the First Council of Nicaea in 325. The names of many of his successors in the first millennium are known from extant contemporary documents. At first a suffragan of Nicomedia, it soon became an autocephalous archdiocese, being listed as such in Notitiae Episcopatuum from the 7th century onward. No longer a residential bishopric, Cius is today listed by the Catholic Church as a titular see.
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 is found.