Mascoutah is a city in St. Clair County, Illinois, United States. The population was 8,754 at the 2020 census. It is part of the Metro East region of Greater St. Louis. The city was named for the Mascoutens, a tribe of Algonquian-speaking Native Americans.

St. Clair County is the ninth most populous county in Illinois. Located directly east of St. Louis, the county is part of the Metro East region of the Greater St. Louis metropolitan area in southern Illinois. As of the 2020 United States census, St. Clair County had a population of 257,400, making it the third most populous county in Illinois outside the Chicago metropolitan area. Belleville is the county’s seat and largest city.