East St. Louis is a city in St. Clair County, Illinois, United States, directly across the Mississippi River from St. Louis, Missouri, in the Metro-East region of Southern Illinois. As of the 2010 census, the city had a total population of 27,006, less than one-third of its peak of 82,366 in 1950. Like many larger industrial cities, it has been severely affected by loss of jobs in the restructuring of the railroad industry and de-industrialization of the Rust Belt in the second half of the 20th century. In 1950, East St. Louis was the fourth largest city in Illinois.