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. 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 globalization and the movement of U.S. manufacturing to overseas markets. 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.

In 1994, the Gateway Geyser was added to the city's waterfront, facing the St. Louis Arch. On the grounds of Malcolm W. Martin Memorial Park, it was the world's second-tallest fountain. Designed to complement the Gateway Arch in St. Louis, it shot water to a height of 630 feet (190 m), the same height as the arch. The geyser was closed in 2023 due to costs of refurbishment and to prepare the park for being handed over to the National Park Service as an extension of the Gateway memorial.

Native Americans had long inhabited both sides of the Mississippi River. The Mississippian culture rulers organized thousands of workers to construct complex earthwork mounds at what later became St. Louis and East St. Louis. The center of this culture was the urban complex of Cahokia, located to the south of present-day East St. Louis within Collinsville, Illinois. Before the Civil War, settlers reported up to 50 mounds in the area that became East St. Louis, but most were lost to 19th-century development and later roadbuilding.

East St. Louis lies within the fertile American Bottom area of the present day Metro East area of St. Louis, Missouri. This name was given after the United States acquired the Louisiana Purchase in 1803, and more European Americans began to settle in the area. The village was first named "Illinoistown".

East St. Louis was founded in 1797 by Captain James Piggott, a Revolutionary War veteran. In that year Piggott began operating a ferry service across the Mississippi River, connecting Illinoistown with St. Louis, which had been founded by ethnic French families. When Piggott died in 1799, his widow sold the ferry business, moved to St. Louis County and remarried. One of the Piggotts' great-great-granddaughters became known as actress Virginia Mayo (Virginia Clara Jones).

The municipality called East St. Louis was established on April 1, 1861. Illinoistown residents voted on a new name that day, and 183 voted to rename the town East St. Louis. Though it started as a small town, East St. Louis soon grew to a larger city, influenced by the growing economy of St. Louis, which in 1870 was the fourth-largest city in the United States.

Extensive industrial growth followed the American Civil War, helped by the city's proximity to coal mines in Illinois. Early industry included meatpacking and stockyards, concentrated in one area to limit their nuisance to other jurisdictions.

Many businessmen became overextended in credit, and a major economic collapse followed the Panic of 1873. This was due to railroad and other manufacturing expansion, land speculation, and general business optimism caused by large profits from inflation. The economic recession began in the East and steadily moved West, crippling the railroads, the main system of transportation. In response, railroad companies began dramatically lowering workers' wages, forcing employees to work without pay, and cutting jobs and paid work hours. These wage cuts and additional money-saving tactics prompted strikes and massive unrest.