Jefferson County is a county located in the U.S. state of Washington. As of the 2020 census, the population was 32,977. The county seat and only incorporated city is Port Townsend. The county is named for Thomas Jefferson. Jefferson County was formed out of Thurston County on December 22, 1852, by the legislature of Oregon Territory, and included the northern 4,854-square-mile portion of the Olympic Peninsula.