Kelso is a city in the southwestern part of the U.S. state of Washington and is the county seat of Cowlitz County. At the 2020 census, the population was 12,720. Kelso is part of the Longview, Washington Metropolitan statistical area, which has a population of 110,730. Kelso shares its long western border with Longview. It is near Mount St. Helens.

Cowlitz County is a county located in the U.S. state of Washington. As of the 2020 census, its population was 110,730. The county seat is Kelso, and its largest city is Longview. The county was formed in April 1854. Its name derives from the anglicized version of the Cowlitz Indian term Cow-e-liske, meaning either 'river of shifting sands' or 'capturing the medicine spirit.' Cowlitz comprises the Longview, WA Metropolitan statistical area, which is also included in the Portland-Vancouver-Salem, OR-WA Combined statistical area.