San Juan Island is the second-largest and most populous of the San Juan Islands in the Pacific Northwest. The land area is 142.59 km² (55.053 sq mi) and has 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, and runs from Sidney, BC in Canada and Anacortes, WA. The only major commercial establishment outside of Friday Harbor is the village and resort of Roche Harbor on the northwest side of the island.

Scheduled seaplane services operate regularly to Friday Harbor from Seattle Lake Union and Kenmore, WA.

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