| Koyun Island | |
| Native Name: | Koyun Adası |
| Native Name Lang: | Turkish |
| Map: | Turkey |
| Coordinates: | 40.5092°N 27.5778°W |
| Location: | Marmara |
| Country: | Turkey |
| Country Admin Divisions Title: | İl (province) |
| Country Admin Divisions: | Balıkesir Province |
| Country Admin Divisions Title 1: | İlçe |
| Country Admin Divisions 1: | Bandırma |
Koyun Island (Turkish: Koyun Adası, literally "Sheep Island") is a Turkish island in the Sea of Marmara. At 40.5092°N 27.5778°W it is a part of Erdek ilçe (district) of Balıkesir Province.[1] 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 . There are only a few summer houses on the otherwise uninhabited island.[2]