San Juan County is a county in the Salish Sea in the far northwestern corner of the U.S. state of Washington. As of the 2020 census, its population was 17,788. The county seat and only incorporated town is Friday Harbor, on San Juan Island. The county was formed on October 31, 1873, from Whatcom County and is named for the San Juan Islands, which are in turn named for Juan Vicente de Güemes, 2nd Count of Revillagigedo, the Viceroy of New Spain.

Friday Harbor is a town in San Juan County, Washington, United States. The population was 2,613 at 2020 census. Located on San Juan Island, Friday Harbor is the major commercial center of the San Juan Islands archipelago and is the county seat of San Juan County.