Shelby County is a county located in the U.S. state of Illinois. According to the 2020 census, it had a population of 20,990. Its county seat is Shelbyville.

Shelbyville is a city in and the county seat of Shelby County, Illinois, United States, along the Kaskaskia River. As of the 2020 census, the population was at 4,674. HSHS Good Shepherd Hospital, located in town, is the county's only hospital. Shelbyville is also home to Chautauqua Auditorium.