Salem is a city in and the county seat of Marion County, Illinois, United States. The population was 7,282 at the 2020 census. Salem was fou…Salem is a city in and the county seat of Marion County, Illinois, United States. The population was 7,282 at the 2020 census. Salem was founded in 1823 as the county seat of the newly formed Marion County. It is situated halfway between the Indiana and Missouri borders on what was originally the Vincennes-St. Louis Road, now U.S. Highway 50.