A One-game playoff is an extra game tacked on to the end of the regular season to determine which of two teams who have finished with the same record get to advance to the postseason. Statistics from such playoff games are counted with regular-season statistics. Until 2011, a playoff was only used when there was only one spot available in the postseason for the two teams; in cases where one team was going to make the postseason as the division winner and the other as the wild card, tiebreakers were used to determine which team earned which spot. This changed with the introduction of the Wild Card Game, as winning a division title became a much more significant prize. The first time that a one-game playoff was staged between two teams already qualified for the postseason was in 2018, when two such games took place on the same day, both between National League teams, with the winners both getting home field advantage in the Division Series, and the two losing teams headed to the Wild Card Game the next day.
The American League has always used a one-game playoff to settle who will advance in these situations. Prior to the advent of divisional play in 1969, the National League used a three-game playoff to settle entry into the World Series. With the adoption of divisions, however, the NL switched to a one-game playoff format as well.
Starting in 2022, with the adoption of a twelve-team postseason format, the possibility of playing a one-game playoff no longer exists: all ties will now be determined by a tiebreaker formula, something which is likely to bring significant controversy as soon as one team is excluded from the postseason thanks to a mathematical formula.
American League one-game playoffs:
- 1948 American League: Cleveland Indians v. Boston Red Sox
- 1978 AL East: New York Yankees v. Boston Red Sox
- 1995 AL West: California Angels v. Seattle Mariners
- 2008 AL Central: Minnesota Twins v. Chicago White Sox
- 2009 AL Central: Minnesota Twins v. Detroit Tigers
- 2013 AL Wild Card: Tampa Bay Rays v. Texas Rangers
National League one-game playoffs:
- 1980 NL West: Houston Astros v. Los Angeles Dodgers
- 1998 NL Wild Card: San Francisco Giants v. Chicago Cubs
- 1999 NL Wild Card: Cincinnati Reds v. New York Mets
- 2007 NL Wild Card: San Diego Padres v. Colorado Rockies
- 2018 NL Central: Milwaukee Brewers v. Chicago Cubs
- 2018 NL West: Colorado Rockies v. Los Angeles Dodgers
- Matt Kelly, Daniel Kramer and Chad Thornburg: "A complete breakdown of MLB tiebreaker games", mlb.com, September 30, 2018. 
- Evan Thompson: A Complete History of the Major League Baseball Playoffs - Volume I: Pre-divisional tiebreakers through 1976, BookBaby, Pennsauken Township, NJ, 2021, ISBN 9781098372804