Island County is a county located in the U.S. state of Washington. As of the 2020 census, its population was 86,857. Its county seat is Coupeville, while its largest city is Oak Harbor. The county's name reflects the fact that it is composed entirely of islands. It contains two large islands, Whidbey and Camano, and seven smaller islands. Island County was created out of Thurston County on December 22, 1852, by the legislature of Oregon Territory, and is the eighth-oldest county in Washington.