St. Louis () is a city in the U.S. state of Missouri, on the western bank of the Mississippi River, which forms the state line between Illinois and Missouri. The Missouri River merges with the Mississippi River 15 river miles north of Downtown St. Louis, forming the fourth-longest river system in the world. In 2019, the estimated population was 300,576, and of the bi-state metropolitan area, 2,804,724. Greater St. Louis is the largest metropolitan area in Missouri, second-largest in Illinois, seventh-largest in the Great Lakes Megalopolis, and the 20th-largest in the United States.
Before European settlement, the area was a regional center of Native American Mississippian culture. The city of St. Louis was founded in 1764 by French fur traders Pierre Laclède and Auguste Chouteau, and named after Louis IX of France.