|No. of teams||12|
|Last champion(s)||1884-St. Louis Maroons|
The Union Association was a league in Major League Baseball which lasted for only 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.
Although the league is conventionally listed as a major league, this status has been questioned by a number of modern baseball historians, most notably Bill James in The Bill James Historical Baseball Abstract. The league had a number of major league players (on the St. Louis franchise, at least), but the league's overall talent and organization was notably inferior to that of the two established major leagues. James found that the contemporary Baseball Guides didn't consider the Union Association to be a major league either. The earliest record he could find of the Union Association as a major league was Ernest Lanigan's "The Baseball Cyclopedia", published in 1922.
For example, the league's only "star" player, Fred Dunlap, led the league in batting average with .412 (86 points higher than his second-best season and 120 points higher than his career average), on-base percentage, slugging percentage, runs scored, hits, total bases, and home runs (with just 13, typical for the era). After the league folded, Dunlap never hit more than .274 or more than 10 home runs until he retired in 1891, another measure of the inferior quality of the Union Association.
Of the 272 players in the Associaton, 107 (39.34%) never played in another major league, while 72 (26.47%) played very briefly (less than 300 at bats and/or 50 hits) in other major leagues, and 79 (29.04%) had longer careers but little success in other major leagues. 
A relatively modern comparison could be the World Football League of the early 1970s contrasted with the National Football League. The WFL similarly resorted to putting clubs in small cities or cities with established teams, and collapsed in the middle of a season.
The league was founded in September 1883 by the young St. Louis millionaire Henry Lucas. Lucas was eventually named the league's president, with owner Tom Pratt of the Philadelphia franchise serving as vice-president and Warren W. White of the Washington franchise as secretary.
Lucas' favoritism toward his own team doomed the league from the beginning, as acquired the best available players for his St. Louis franchise at the expense of the rest of the league, and the Maroons won 94 games while losing only 19, for an .832 percentage; for comparison, the Maroons' record would project to 135-27 under the modern schedule of 162 games, while Pythagorean expectation based on the Maroons' results (887 runs scored, 452 runs allowed) and a 162-game schedule would translate to a record of 131-31, but these results are of questionable merit.
The lopsided competition, the revolving-door nature of the franchises and the poorly drafted schedule was also a major problem; four franchises folded during the season, forcing the league to scramble to replace them with three teams from lower classification leagues and one new team, and the league was derisively dubbed "The Onion League" by its detractors in the two established leagues.
The Altoona team was the first to fold in May, and was replaced by a newly-formed team in Kansas City. After the Philadelphia franchise folded in August, the Unions recruited the Wilmington Quicksteps from the Eastern League, but the Quicksteps lost four of their star players, and dropped out of the Association and folded in September. The Chicago franchise moved to Pittsburgh in August and finally disbanded about the same time as Wilmington, and both teams were replaced by two teams from the disbanded Northwest League, Milwaukee and St. Paul. On January 15, 1885, at a scheduled UA meeting in Milwaukee, only the Milwaukee and Kansas City franchises showed up, and the league was promptly disbanded.
The St. Louis franchise itself was deemed to be strong enough to enter the National League in 1885, but it faced heavy competition within the city, as the St. Louis Browns were a power in the American Association. The lone survivor of the Union moved to Indianapolis and became the Hoosiers after 1886, having compiled records of 36-72 and 43-79, and played three seasons before folding, with records of 37-89, 50-85 and 59-75 in NL play for an overall win percentage in the NL of .360. These figures perhaps reveal the gulf in class between the UA and the established major leagues.
Perhaps the most obvious impact of the short-lived league was on the career of a player who did not jump to the new league: Charles Radbourn. With a schedule of a little over 100 games, most teams employed two regular pitchers, and the Providence Grays in the National League featured Radbourn and Charlie Sweeney. According to the book Glory Fades Away, by Jerry Lansche, Sweeney fell out of grace with the Providence team in late July after he refused to be replaced in a game while drunk, and was expelled. Rather than come crawling back, Sweeney signed with Lucas' team, leaving Radbourn by himself.
Leveraging his situation, Radbourn pledged to stay with the club and be the sole primary pitcher if he would be given a raise and granted free agency at season's end. Radbourn, who already had 24 wins at that point to Sweeney's 17, pitched nearly every game after that, and went on to win an astounding 60 games (a record) during the regular season. For an encore, he won all three games of 1884's version of the World Series, pitching every inning of a sweep of the New York Metropolitans of the American Association. His performance in 1884, along with a generally strong career, and being the first pitcher to reach 300 wins, assured his place in the Baseball Hall of Fame.
Notable players 
Notable players that made their debut in the Union Association included Tommy McCarthy, who was elected to the Hall of Fame in 1946, and Jack Clements, the only man in baseball history to play a full career as a left-handed catcher.
The Union Association saw two no-hitters in its brief existence: one by Dick Burns of the Outlaw Reds on August 26 and one by Ed Cushman of the Brewers on Sept. 28. On July 7, Hugh Daily struck out 19 Boston Reds in a nine-inning game, an "MLB" record that would stand for 102 years, until Roger Clemens struck out 20 batters in a game in 1986. Henry Porter and Dupee Shaw got 18-strikeout games. The Chicago Browns executed a triple play on June 19.
|St. Louis Maroons||94||19||0.832||—||49–6||45–13|
|Cincinnati Outlaw Reds||69||36||0.657||21||35–17||34–19|
|St. Paul Saints||2||6||0.250||39½||0–0||2–6|
|Chicago Browns/Pittsburgh Stogies||41||50||0.451||42||21–19||20–31|
|Altoona Mountain City||6||19||0.240||44||6–12||0–7|
|Washington Nationals (UA)||47||65||0.420||46½||36–27||11–38|
|Kansas City Cowboys||16||63||0.203||61||11–23||5–40|
- Baseball Prospectus | Unfiltered
- The Chronology - 1883 | BaseballLibrary.com
- Richter, Francis C. (March 14). "Two Big Wars Interrupted the Progress of the National Game". Sporting Life 51 (1): 6. Retrieved 21 September 2011.
- The Chronology - 1885 | BaseballLibrary.com
- 1884 Union Association Baseball Debuts / Rookies by Baseball Almanac
- David Pietrusza Major Leagues: The Formation, Sometimes Absorption and Mostly Inevitable Demise of 18 Professional Baseball Organizations, 1871 to Present Jefferson (NC): McFarland & Company, 1991. ISBN 0-89950-590-2
- Union Association at baseball-reference.com.
- Union Association and 1884 in baseball at baseballlibrary.com