The Basilica of Saint Louis, King of France, formerly the Cathedral of Saint Louis, and colloquially the Old Cathedral, is a Catholic church in St. Louis, Missouri. It was the first cathedral west of the Mississippi River and until 1844 the only parish church in St. Louis. It is one of two Catholic basilicas in St. Louis and both are named for King Louis IX of France.

St. Louis is an independent city in the U.S. state of Missouri. It is located near the confluence of the Mississippi and the Missouri rivers. In 2020, the city proper had a population of 301,578, while its bi-state metropolitan area, which extends into Illinois, had an estimated population of over 2.8 million. It is the largest metropolitan area in Missouri and the second largest in Illinois. The city's combined statistical area (CSA) is the 20th largest in the United States.