DuPage County is a county in the U.S. state of Illinois, and one of the collar counties of the Chicago metropolitan area. As of the 2020 census, the population was 932,877, making it Illinois' second-most populous county. Its county seat is Wheaton. Known for its vast tallgrass prairies, DuPage County has become mostly developed and suburbanized, although some pockets of farmland remain in the county's western and northern parts.