St. Louis is an independent city and major U.S. port in the state of Missouri, built along the western bank of the Mississippi River, which marks Missouri's border with Illinois. The city had an estimated March 22, 2018 population of 308,626 and is the cultural and economic center of the Greater St. Louis Metropolitan area, which is the largest metropolitan area in Missouri and the 19th-largest in the United States.
Prior to European settlement, the area was a major 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. In 1764, following France's defeat in the Seven Years' War, the area was ceded to Spain and retroceded back to France in 1800. In 1803, the United States acquired the territory as part of the Louisiana Purchase. During the 19th century, St. Louis developed as a major port on the Mississippi River. In the 1870 Census, St. Louis was ranked as the 4th-largest city in the United States. It separated from St. Louis County in 1877, becoming an independent city and limiting its own political boundaries.