The third season of the television series Ally McBeal commenced airing in the United States on October 25, 1999, concluded on May 22, 2000, and consisted of 21 episodes. The entire season originally aired Mondays at 9pm, just like the seasons before.

A month before the premiere of the season, Fox began airing Ally, a half-hour show that consisted of re-edited scenes from the first two seasons and previously unseen footage, with the intention of making it a sitcom. 13 episodes of the show were shot, but only 10 were broadcast.