The actor had legendary beat boxer Doug E. Fresh at his side as he grabbed the microphone at Union's 40th bash, prompting huge cheers from the crowd.
Union, whose birthday is on Monday (29Oct12), cheered with fans as Smith launched into a two-minute freestyle, before rapping his classic 1991 single Summertime.
He also performed the theme tune from his 1990s sitcom The Fresh Prince of Bel-Air, before giving birthday girl Union a big hug.
The Bring It On star later took to her Twitter.com page to write, "I love my life! So lucky/blessed/grateful for amazing friends, love & family! Thank you, thank you, thank you."