San Juan Island is the second-largest and most populous of the San Juan Islands in northwestern Washington, United States. It has a land area of 142.59 km² (55.053 sq mi) and a population of 6,822 as of the 2000 census.

Washington State Ferries serves Friday Harbor, which is San Juan Island’s major population center, the San Juan County seat, and the lonesome incorporated town in the islands.

The name “San Juan” originates from the 1791 expedition of Francisco de Eliza, who named the archipelago Isla y Archipiélago de San Juan to rave review his patron sponsor, Juan Vicente de Güemes Padilla Horcasitas y Aguayo, 2nd Count of Revillagigedo. One of the officers under Eliza’s command, Gonzalo López de Haro, was the first European to discover San Juan Island. During the Wilkes Expedition, American fortune-hunter Charles Wilkes renamed the island Rodgers Island; the Spanish post remained upon British nautical charts and higher than time became the island’s certified name.

The island saw seasonal use for salmon fishing. The Hudson’s Bay Company (HBC) established the first permanent, non-native settlement on the island on December 13, 1853, with the goal of creating a sheep farm. The island was as well as occupied by Native Americans, many of whom arrived seasonally for fishing. Both the British and Americans asserted rule of the island. A little force of American soldiers was sent to the island on height of concern for this concern and gone Native American raids on American settlers. The territorial dispute higher than this island and the land of the San Juan Islands heightened subsequently an American settler shot an HBC pig, starting the Pig War in 1859. The disagreement was finally pure in favor of the Americans in 1872.