Clark County is the southernmost county in the U.S. state of Washington. As of the 2020 census, the population was 503,311, making it Washington's fifth-most populous county. Its county seat and largest city is Vancouver. It was the first county in Washington, first named Vancouver County in 1845 before being renamed for William Clark of the Lewis and Clark Expedition in 1849. It was created by the Provisional Government of Oregon in Oregon Country on August 20, 1845, and at that time covered the entire present-day state. Clark County is the third-most-populous county in the Portland-Vancouver-Hillsboro Metropolitan Statistical Area, and is across the Columbia River from Portland, Oregon.

Vancouver is a city on the north bank of the Columbia River in the U.S. state of Washington, located in Clark County. Founded in 1825 and incorporated in 1857, Vancouver had a population of 190,915 as of the 2020 census, making it the fourth-most populous city in Washington state. Vancouver is the seat of government of Clark County and forms part of the Portland-Vancouver metropolitan area, the 25th-largest metropolitan area in the United States. Originally established in 1825 around Fort Vancouver, a fur-trading outpost, the city is located on the Washington–Oregon border along the Columbia River, directly north of Portland, and is considered a suburb of the city along with its surrounding areas.