Arapahoe County is one of the 64 counties in the U.S. state of Colorado. As of the 2010 census, the population was 572,003, 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, CO Metropolitan Statistical Area.