Benton County is a county in the south-central portion of the U.S. state of Washington. As of the 2020 census, its population was 206,873. The county seat is Prosser, and its largest city is Kennewick. The Columbia River demarcates the county's north, south, and east boundaries. Benton County was created from what were then larger versions of Klickitat County and Yakima County on March 8, 1905, and was named after Missouri statesman Thomas Hart Benton.