  Shown within Turkey | |
| Location | Turkey |
|---|---|
| Region | Bursa Province |
| Coordinates | 40°25′57″N 29°09′23″E / 40.432468°N 29.156389°E |
Cius (/ˈsaɪəs/; 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.