Ōmishima Island (大三島, Ōmishima) is the largest island in the Geiyo Islands chain, and the westernmost island on which the Nishiseto Expressway, which links Honshu and Shikoku via a number of islands, runs. 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).

The population as of the 2020 census was 4,963 - 2,299 men, and 2,264 women.

Ōmishima is kidney-shaped and together with Ōsakikamijima on the west encloses the calm bay of Utena, where the primary seaport of Miyaura [ja] is located. The island's Utena Dam reservoir is the primary freshwater source for Ōmishima itself and the nearby Hakata.

• 1541 - Tsuruhime fights Ōuchi Yoshitaka fleet
• 1874 - post office established
• 1979 - Ōmishima bridge [ja] connecting to Ehime Prefecture complete
• 1999 - Tatara Bridge connecting to Hiroshima Prefecture complete
• 16 January 2005 - several towns (including Ōmishima, Ehime) and villages on the island were merged into the city of Imabari, Ehime, along with towns and villages on other surrounding islands.

Ōmishima is connected to the mainland of Honshu and Shikoku islands by the bridges of the Nishiseto Expressway (Shimanami Kaidō). A ferry is also available from the island to Ōsakikamijima, Okamura Island and Ōkunoshima (Rabbit Island). Ōmishima Island is served by the national Route 317.

• The salt factory
• Historical and marine museum
• Ōyamazumi Shrine
• Toyo Ito Museum of Architecture, Imabari
• The entire Ōmishima Island is designated as a National Place of Scenic Beauty

• Shozo Fujita - historian
• Tsuruhime - priestess and warlord

• Imabari city official site

• This article incorporates material from Japanese Wikipedia pages 大三島 and 大三島町, accessed 13 September 2017