The Umbrella singer - who doesn’t turn 20 until Wednesday - was partying with friends including Kanye West and actor Wilmer Valderrama at Los Angeles’ Les Deux nightclub when the food fight began on Tuesday night.
An onlooker said: “There was cupcake frosting all over Rihanna’s face and the group was hysterically laughing to the point of almost tears!”
Chris Brown, who is rumoured to be dating Rihanna, was later given a standing ovation after he sang her an acapella version of Happy Birthday.
Despite Chris insisting he and Rihanna are “just friends”, onlookers at the exclusive club said the pair were “cuddling and acting very cosy all night”.