Boise is the capital and most populous city of the U.S. state of Idaho. Boise is the county seat of Ada County. The population of the city was 235,685 at the 2020 census. The Boise metropolitan area, also known as the Treasure Valley, includes five counties of Idaho with an estimated population of 846,000, the most populous metropolitan area in Idaho and 74th-most populous in the United States.