Boise) is the capital and most populous city of the U.S. state of Idaho, and is the county seat of Ada County. Located on the Boise River in southwestern Idaho, the population of Boise at the 2010 Census was 205,671, the 99th largest in the United States. Its estimated population in 2016 was 223,154.The Boise-Nampa metropolitan area, also known as the Treasure Valley, includes five counties with a combined population of 664,422, the most populous metropolitan area in Idaho. It contains the state's three largest cities; Boise, Nampa, and Meridian. Boise is the third most populous metropolitan area and city in the United States' Pacific Northwest region, behind Seattle and Portland.