East St. Louis is a city in St. Clair County, Illinois, United States. It is directly across the Mississippi River from Downtown St. Louis 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 loss of jobs due to industrial restructuring during the second half of the 20th century. In 1950, East St. Louis was the fourth-largest city in Illinois when its population peaked at 82,366. As of the 2010 census, the city had a population of 27,006, less than one-third of the 1950 census.