The Union Association was a league in Major League Baseball which lasted only for one season in 1884. St. Louis won the pennant and joined the National League the following season. Chicago moved to Pittsburgh in late August, and four teams folded during the season and were replaced.

Union Association franchises

• Altoona Unions
• Baltimore Monumentals
• Boston Unions
• Chicago Browns / Pittsburgh Stogies
• Cincinnati Outlaw Reds
• Keystones of Philadelphia
• Washington Nationals
• St. Louis Maroons

Midseason replacement teams

• Kansas City Unions replaced Altoona
• Wilmington Quicksteps replaced Philadelphia
• Milwaukee Grays replaced Wilmington
• St. Paul Whitecaps replaced Pittsburgh