Champaign County is a county in the U.S. state of Illinois. As of the 2020 Census, its population was 205,865, making it the 10th-most populous county in Illinois. Its county seat is Urbana. Champaign County is part of the Champaign–Urbana metropolitan area. The twin cities of Urbana and Champaign are the only cities in the county, and they nearly surround the campus of the University of Illinois.