Smith, 46, who was at the show to present an award to his wife, actress Jada Pinkett Smith, ran into his co-star Ali backstage at the show in Newark, New Jersey.
Ali, 36, played Smith's cousin Ashley Banks on the popular sitcom from 1990 to 1996.
The actress shared a quick selfie of the duo on Instagram, writing "Backstage @blackgirlsrock @mbib #imagineafuture #nofilter #fam #tatyanaali #willsmith #FRESHPRINCE."
The TV cousins have been reunited before, with Ali joining another "Fresh Prince" star Alfonso Ribeiro at the premiere of Smith's 2008 film "Seven Pounds".