Thurston County is a county located in the U.S. state of Washington. As of the 2020 census, its population was 294,793. The county seat and largest city is Olympia, the state capital. Thurston County was created out of Lewis County by the government of Oregon Territory on January 12, 1852. At that time, it covered all of the Puget Sound region and the Olympic Peninsula. On December 22 of the same year, Pierce, King, Island, and Jefferson counties were split off from Thurston County.