One US Bank Plaza (formerly One Mercantile Center) is a 36-story building in Downtown St. Louis, Missouri. The building is topped by an antenna that raises the total building to . In the 1990s the Ambassador Building next to it was razed and became part of the building's plaza.
The building has a Structural Expressionism style. It was originally built for Mercantile Bancorporation which was bought out by Firstar in 1999 and then became U.S. Bancorp in 2001.