St. Louis (English /seɪnt ˈluːɪs/, French /sɛ̃ lwi/) is an independent city in the U.S. state of Missouri. It is bordered by the Mississippi River on the east and by St. Louis County on the north, south, and west. St. Louis is the largest metropolitan area in Missouri. Sometimes written as Saint Louis, the city is named for King Louis IX of France. St. Louis is famous for its multiple French and German influences as well as having a Victorian past. Two events at the beginning of the 20th century, the 1904 World's Fair and 1904 Olympic Games (the first ever held in the United States) are of particular pride to St. Louisans. In the 21st century, St. Louis has transformed from a manufacturing and industrial economy into a globally known focus for research in medicine, biotechnology, and other sciences.

The city has many nicknames, the most popular being "Gateway City", as it is seen as the Eastern/Western US dividing mark. St. Louis is also called "Gateway to the West" on behalf the many people who moved west through St. Louis via the Missouri River (first leg of the Oregon Trail) and other wagon trails. St. Louis is also called "Mound City". This term originated with the Native American burial mounds that once were common in the city. These were largely destroyed to level the ground as the urban area grew. The city has several modern names such as "The Lou", "St. Louie", "The Western-Most Eastern City", "River City", or "The 314" (reference to the area code for St. Louis center). The most popular abbreviation for St. Louis is the "STL" in reference to the airport code for the city and the long-standing use of an interlocked S, T, and L by the St. Louis Cardinals baseball team (the St. Louis Browns also used an interlocked STL).

The City of St. Louis lies at the heart of Greater St. Louis, a Combined Statistical Area of nearly 3,000,000 people in both Missouri and Illinois. The Greater St. Louis area was the 16th largest urbanized area in the U.S. as of the July 2006 US Census estimate, with 2,858,549 people.