The Boise, Idaho Metropolitan Statistical Area (MSA) is an area that encompasses Ada, Boise, Canyon, Gem, and Owyhee counties in southwestern Idaho, anchored by the cities of Boise and Nampa. It is the main component of the wider Boise–Mountain Home–Ontario, ID–OR Combined Statistical Area, which adds Elmore and Payette counties in Idaho and Malheur County, Oregon. It is the state's largest officially designated metropolitan area and includes Idaho's three largest cities: Boise, Nampa, and Meridian. Nearly 40 percent of Idaho's total population lives in the area.

Ada County is located in the southwestern part of Idaho, United States. As of the 2020 census, the county had a population of 494,967, which by 2022 was estimated to have risen to 518,907. Ada County is by far the state's most populous county; it is home to 26.8% of the state's population. The county seat and largest city is Boise, which is also the state capital. Ada County is included in the Boise metropolitan area. The Ada County Highway District has jurisdiction over all the local county and city streets, except for private roads and state roads. In the interior Pacific Northwest east of the Cascade Range, Ada County ranks second in population, behind Spokane County, Washington.