Boise is the capital and most populous city in the U.S. state of Idaho and is the county seat of Ada County. As of the 2020 census, there were 23…Boise is the capital and most populous city in the U.S. state of Idaho and is the county seat of Ada County. As of the 2020 census, there were 235,684 people residing in the city. On the Boise River in southwestern Idaho, it is 41 miles east of the Oregon border and 110 miles north of the Nevada border. The downtown area's elevation is 2,704 feet above sea level.