UFC 21: Return of the Champions was a mixed martial arts event held by the Ultimate Fighting Championship on July 16, 1999 at the Five Seasons Events Center in Cedar Rapids, Iowa.

The event was seen live on pay per view in the United States, and later released on home video.

UFC 21 marked significant rule changes due to the Council of the Mixed Martial Arts Commission: preliminary bouts now consisted of two five minute-rounds, main card bouts were three five-minute rounds and championship bouts were five five-minute rounds (similar to the Unified Rules of Mixed Martial Arts used today in the United States).

Points are awarded for Octagon control, effective striking, grappling/near submissions, and aggressiveness.

[1] During the event, it was announced that UFC Middleweight Champion Frank Shamrock would defend his title at UFC 22 against rising star Tito Ortiz.