Millstadt is a village in St. Clair County, Illinois, United States, located at the crossing of Illinois Routes 163 and 158. The village is known for its German heritage, with more than half its people of German descent. The population was 4,011 at the 2010 census, but a more recent study in July 2018 estimated the number at 3,885.

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.