Moultrie County is a county in the U.S. state of Illinois. According to the 2020 United States Census, its population was 14,526. Its county seat is Sullivan. The name is pronounced as in "mole tree", unlike the pronunciation of its namesake, the South Carolinian Revolutionary War hero William Moultrie.

Sullivan is a city in Moultrie County, Illinois, United States. The population was 4,413 at the time of the 2020 census. It is the county seat and largest city of Moultrie County. Sullivan is named after Sullivan's Island, South Carolina, where Fort Moultrie is located.