Randolph County is a county located in the U.S. state of Illinois. According to the 2020 census, it had a population of 30,163. Its county seat is Chester.

Chester is a city in and the county seat of Randolph County, Illinois, United States, on a bluff above the Mississippi River. The population was 7,640 at the 2020 census. It lies 61 miles (98 km) south of St. Louis, Missouri.