Ada County is located in the southwestern part of Idaho, United States. As of the 2020 United States 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.