Willow Smith was a surprise guest at Justin Bieber's sold-out show at Los Angeles' Staple Center on October 25.

The 'Whip My Hair' star grabbed the microphone and performed her debut single with Bieber, who is best mates with her brother, Jaden Smith.

The two youngsters also danced freestyle together to raptuous applause at the concert, which is part of Bieber's on-going 'My World' tour. 

Bieber twittered post show: "LA TONIGHT WAS CRAZY!!! USHER, JADEN, SEAN, and even my lil sis WILLOW SMITH!!!! WE SHUT LA DOWN!!!!!!!! LOVE YALL."

Watch footage of the pair together onstage at the concert below: