The Tennis Masters Cup is a tennis tournament played at the end of each year, involving the top eight players in the men's tennis world rankings.

Unlike most of the other events on the men's tour, the Tennis Masters Cup is not a straightforward knock-out tournament. The eight players are divided into two groups of four, and play three round-robin matches each against the other three players in their group. From there, the two players with the best records in each group progress to the semifinals, with the winners meeting in the final to determine the champion.

The event is actually the third evolution of a championship which began in 1970. It was originally known simply as The Masters tournament, and was organised by the International Tennis Federation (ITF). The Masters was a year-end showpiece event between the best players on the men's tour, but did not count for any world ranking points. In 1990, the Association of Tennis Professionals (ATP) took over the running of the men's tour and replaced the Masters with the ATP Tour World Championship. World ranking points were now at stake, with an undefeated champion earning the same number of points they would for winning one of the four Grand Slam events. The ITF, who continued to run the Grand Slam tournaments, created a rival year-end event known as the Grand Slam Cup, which was contested by the 16 players with the best records in Grand Slam competitions that year. In December 1999, the ATP and ITF agreed to discontinue the two separate events and create a new jointly-owned event called the Tennis Masters Cup. As with the Masters and the ATP Tour World Championships, the Tennis Masters Cup is contested by eight players. However under the rules of the Tennis Masters Cup, the player who is ranked number eight in the ATP Champion's Race world rankings does not have a guaranteed spot. If a player who wins one of the year's Grand Slam events finishes the year ranked outside the top eight but still within the top 20, he may be included in the Tennis Masters Cup instead of the eighth-ranked player. If two players outside the top eight win Grand Slam events, the higher placed player in the world rankings will take the final spot in the Tennis Masters Cup.

In 2009 the Masters will be renamed the ATP World Tour Finals and it will be staged at The O 2 in London from 2009 to 2012 .

For many years, the doubles event was held as a separate tournament the week after the singles competition. But more recently they have been held together in the same week and venue. Like the singles competition, the doubles involves the eight most successful teams on the tour each year and starts with a group phase with each team playing three round-robin matches.

Former World No.1s Pete Sampras and Ivan Lendl hold the record for the most titles, each with 5.