| Ōmishima Island | |
| Map: | Japan |
| Native Name: | 大三島 Ōmishima |
| Native Name Link: | Japanese language |
| Location: | Seto Inland Sea |
| Coordinates: | 34.2441°N 133.0096°W |
| Area Km2: | 64.54 |
| Length Km: | 12.8 |
| Width Km: | 6.2 |
| Coastline Km: | 88.8 |
| Highest Mount: | Washigatozan |
| Elevation M: | 437 |
| Country: | Japan |
| Country Admin Divisions Title: | Prefecture |
| Country Admin Divisions: | Ehime |
| Country Admin Divisions Title 1: | City |
| Country Admin Divisions 1: | Imabari |
| Population: | 23398 |
| Population As Of: | 1947 |
| Density Km2: | 363 |
| Ethnic Groups: | Japanese |
is the largest island in the Geiyo Islands chain, and the westernmost which accommodates the Nishiseto Expressway between Honshu and Shikoku. It is located in the Seto Inland Sea. The island's highest peak is at an altitude of 437m (1,434feet).
Ōmishima is kidney-shaped and together with Ōsakikamijima on the west encloses the calm bay of Utena, where the primary seaport of is located. The island's Utena Dam reservoir is the primary freshwater source for Ōmishima itself and the nearby Hakata.
Ōmishima is connected to the mainland of Honshu and Shikoku islands by bridges of Nishiseto Expressway (Shimanami Kaidō). Also, ferry is available to Ōsakikamijima, Okamura Island and Ōkunoshima (Rabbit Island). Ōmishima Island is served by the national Route 317.