Best-of-nine playoff is a sport event which consists of a competition between two teams head-to-head which must win five games to win the series. Five is chosen as it would constitute a majority of the games played. If one team has won five games before all nine games have been played, the games that remain are omitted.

The World Series of baseball was conducted as a best-of-nine playoff in its first year of existence in 1903, then again for three years beginning in 1919, the year of the "Black Sox scandal."

See also:

• Best-of-three playoff
• Best-of-five playoff
• Best-of-seven playoff