Jefferson County is a county located in the U.S. state of Washington, named after Thomas Jefferson. As of the 2010 census, the population was 29,872. The county seat is at Port Townsend, which is also the county's only incorporated city.
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 miles (12,571.8 km2) portion of the Olympic Peninsula. On April 26, 1854, the legislature of Washington Territory created Clallam County from the northwestern 2,670 square miles (6,915.3 km2) portion of this original area.
The Hood Canal Bridge connects Jefferson County to Kitsap County, Washington. The Keystone-Port Townsend route of the Washington State Ferries connects the county to Whidbey Island in Island County, Washington.