|Native name: Koyun Adası|
|İl (province)||Balıkesir Province|
Koyun Island (Turkish: Koyun Adası, literally "Sheep Island") is a Turkish island in the Sea of Marmara. At 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 where the longer dimension is directed from north to south direction. 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.