Clark County is a county located in the southeastern part of U.S. state of Illinois, along the Indiana state line. As of the 2020 census, the population was 15,455. Its county seat is Marshall. The county was named for George Rogers Clark, an officer who served in the American Revolution.

Marshall is a city in and the county seat of Clark County, Illinois, United States, located approximately 20 miles (32 km) west of Terre Haute, Indiana. The population was 3,947 at the 2020 census.