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 / 40.50917; 27.57778 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.