Koyun Island (Turkish: Koyun Adası, literally "Sheep Island") is a Turkish island in the Sea of Marmara. At 40°30′33″N 27°34′40″E / 40.50917°N 27.57778°E it is a part of Erdek ilçe (district) of Balıkesir Province. It is situated between the better known islands of Avşa Island and Paşalimanı Island.
The island is a long island from north to south. Its total area is about 1.7 square kilometres (0.66 sq mi). There are only a few summer houses on the otherwise uninhabited island.
Erdek is a municipality and district of Balıkesir Province, Turkey. Its area is 307 km, and its population is 32,021 (2024). Located on the Kapıdağ Peninsula, on the north coast of the Gulf of Erdek at the south of the Sea of Marmara, Erdek is a popular domestic holiday destination with several hotels dating back to the 1960s. The surrounding area has a rugged geology and topography with evergreen wooded areas and large olive groves. It is dominated by Mt Dindymus (782m). Erdek district also included the islands of Koyun and Paşalimanı.
In the summer ferries travel from Erdek to Avşa Adası, one of the Marmara Islands in the Sea of Marmara. The harbour overlooks tiny Zeytinlik island where there is a research station devoted to olives.