Randolph County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 33,476. Its county seat is Chester.
According to the U.S. Census Bureau, the county has a total area of 597 square miles (1,550 km), of which 576 square miles (1,490 km) is land and 22 square miles (57 km) (3.6%) is water.
The Kaskaskia River flows into the Mississippi River in Randolph County. At this point the Mississippi, which usually defines the border between Illinois and Missouri, is entirely in Illinois. The Mississippi changed its course in the late-nineteenth century, leaving Kaskaskia, the former state capital, on the west side of the river. The boundary of the State, however, follows the old course of the river, leaving Illinois with an exclave on the western shore of the Mississippi River. A smaller enclave, Crains Island, is a few miles further down the river.
As of the 2010 United States Census, there were 33,476 people, 12,314 households, and 8,188 families residing in the county. The population density was 58.2 inhabitants per square mile (22.5/km). There were 13,707 housing units at an average density of 23.8 per square mile (9.2/km). The racial makeup of the county was 87.6% white, 9.7% black or African American, 0.3% Asian, 0.2% American Indian, 1.2% from other races, and 0.9% from two or more races. Those of Hispanic or Latino origin made up 2.6% of the population. In terms of ancestry, 40.3% were German, 11.3% were Irish, 9.4% were English, and 5.7% were American.
Of the 12,314 households, 29.1% had children under the age of 18 living with them, 52.0% were married couples living together, 10.1% had a female householder with no husband present, 33.5% were non-families, and 28.9% of all households were made up of individuals. The average household size was 2.37 and the average family size was 2.90. The median age was 41.0 years.
The median income for a household in the county was $45,020 and the median income for a family was $55,113. Males had a median income of $43,359 versus $28,376 for females. The per capita income for the county was $19,950. About 7.0% of families and 10.4% of the population were below the poverty line, including 11.9% of those under age 18 and 9.9% of those age 65 or over.