Arapahoe County is a county located in the U.S. state of Colorado. As of the 2020 census, its population was 655,070, making it the third-most populous county in Colorado. The county seat is Littleton, and the most populous city is Aurora. The county was named for the Arapaho Native American tribe, who once lived in the region. Arapahoe County is part of the Denver-Aurora-Lakewood metropolitan statistical area.