San Juan County is a county located in the Salish Sea in the far northwestern corner of the U.S. state of Washington. It comprises most of the San Juan Islands. The county is named after San Juan Island, which was named in 1791 by the Spanish explorer Francisco de Eliza for his patron, the Viceroy of New Spain, Juan Vicente de Güemes, 2nd Count of Revillagigedo. The area that now constitutes San Juan County was part of Whatcom County until October 31, 1873. The county seat and the county's only incorporated city is Friday Harbor, on San Juan Island. As of the 2010 census, its population was 15,769. It is the smallest of Washington's 39 counties in land area. It is the only county in Washington not to have state highways.