The Old St. Louis County Courthouse was built as a combination federal and state courthouse in St. Louis, Missouri, United States. Missouri's tallest habitable building from 1864 to 1894, it is now part of Gateway Arch National Park and operated by the National Park Service for historical exhibits and events.

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.