Gemlik is a municipality and district of Bursa Province, Turkey. Its area is 401 km2, and its population is 120,245 (2022). It is located in the east of the Gulf of Gemlik. It is approximately 29 km (18 mi) away from Bursa. In antiquity, Gemlik was the location of the ancient Greek town of Cius.

Cius, later renamed Prusias on the Sea after king Prusias I of Bithynia, was an ancient Greek city bordering the Propontis, in Bithynia and in Mysia, and had a long history, being mentioned by Herodotus, Xenophon, Aristotle, Strabo and Apollonius Rhodius.