Kelso is a city in southwest Washington State, United States, and is the county seat of Cowlitz County. At the 2010 census, the population was 11,925. Kelso is part of the Longview, Washington Metropolitan Statistical Area, which has a population of 102,410. Kelso shares its long western border with Longview, Washington. It is located near Mount St. Helens.
The earliest known inhabitants of Kelso were Native Americans from the Cowlitz tribe. The Cowlitz people were separated into the Upper (or Taidnapam) and Lower (or Mountain) Cowlitz tribes, who were members of the Sahaptin and Salish language families, respectively. In 1855, European explorers noted that there numbered over 6000 individuals of the Cowlitz Tribe.
Kelso was founded by Peter W. Crawford, a Scottish surveyor, who, in 1847, took up the first donation land claim on the Lower Cowlitz River. Crawford platted a townsite which he named after his home town of Kelso, Scotland. The original plat was dated and filed in October 1884. It became incorporated in 1889.