Washington County is one of 36 counties in the U.S. state of Oregon and part of the Portland metropolitan area. The 2020 census recorded the population as 600,372, making it the second most populous county in the state and most populous "Washington County" in the United States. Hillsboro is the county seat and largest city, while other major cities include Beaverton, Tigard, Cornelius, Banks, Gaston, Sherwood, North Plains, and Forest Grove, the county's oldest city. Originally named Twality when created in 1843, the Oregon Territorial Legislature renamed it for the nation's first president in 1849 and included the entire northwest corner of Oregon before new counties were created in 1854. The Tualatin River and its drainage basin lie almost entirely within the county, which shares its boundaries with the Tualatin Valley. It is bordered on the west and north by the Northern Oregon Coast Range, on the south by the Chehalem Mountains, and on the north and east by the Tualatin Mountains, or West Hills.

Oregon is a state in the Pacific Northwest region of the United States. Oregon is a part of the Western United States, with the Columbia River delineating much of Oregon's northern boundary with Washington, while the Snake River delineates much of its eastern boundary with Idaho. The 42° north parallel delineates the southern boundary with California and Nevada. The western boundary is formed by the Pacific Ocean.