Southwestern Idaho is a geographical term for the area along the U.S. state of Idaho's borders with Oregon and Nevada. It includes the populous areas of the Boise metropolitan area and the Treasure Valley.

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.