Ada County is a county in the southwestern part of the U.S. state of Idaho. As of the 2010 Census, the county had a population of 392,365, making it the most populous county in Idaho. The county seat and largest city is Boise, which is also the state capital.
Ada County is included in the Boise, ID Metropolitan Statistical Area.
Ada County is by far the state's largest in population, containing more than one quarter of the state's residents, and contains its only county highway district; the Ada County Highway District (ACHD) has jurisdiction over all the local county and city streets, except for private roads and state roads. In the interior Northwest east of the Cascade Range, Ada County ranks second in population behind only Spokane County, Washington.
Ada County was created by the Idaho Territorial Legislature on December 22, 1864, partitioned from Boise County. It is named for Ada Riggs, the first pioneer child born in the area and the daughter of H.C. Riggs, a co-founder of Boise. Canyon County, which originally included Payette County and most of Gem County, was partitioned from western Ada County in 1891.