Ōmishima Island (大三島, Ōmishima) 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 Washigatozan (鷲ヶ頭山) at an altitude of 437 m (1,434 ft).
Ōmishima is kidney-shaped and together with Ōsakikamijima on the west encloses the calm bay of Utena, where the primary seaport of Miyaura 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.