The 1884 St. Louis Maroons baseball team finished with a 94–19 record and won the championship of the new Union Association (UA). After the season, the UA folded and the Maroons joined the National League; they were the only UA team to continue past the 1884 season.

The St. Louis Maroons were a professional baseball club based in St. Louis, Missouri, from 1884–1886. The club, established by Henry Lucas, were the one near-major league quality entry in the Union Association, a league that lasted only one season, due in large part to the dominance of the Maroons. When the UA folded after playing just one season, the Maroons joined the National League. In 1887 the Maroons relocated to Indianapolis and became the Indianapolis Hoosiers, where they played three more seasons before folding.