East St. Louis or East Saint Louis, also known as ESTL, is a city in St. Clair County, Illinois, United States. It is 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. East St. Louis was Illinois's fourth-largest city in 1950, 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.