East St. Louis is a city in St. Clair County, Illinois, United States. It is directly across the Mississippi River from downtown St. Louis, Missouri, and the Gateway Arch National Park. East St. Louis is in the Metro East region of Southern Illinois. Once a bustling industrial center, like many cities in the Rust Belt, East St. Louis was severely affected by the loss of jobs due to the flight of the population to the suburbs during the riots of the late 1960s. In 1950, East St. Louis was the fourth-largest city in Illinois when its population peaked at 82,366. As of the 2020 census, the city had a population of 18,469, less than one-quarter of the 1950 census and a decline of almost one third since 2010.

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.