Will Ferrell isn't the only one who wants "More cowbell!"

Justin Bieber pranked his tour mate Willow Smith on stage during a concert Sunday night -- and Ryan Seacrest posted video of the hilarious stunt.

While singing her single "Whip My Hair," Bieber's manager Scooter Braun and "swagger coach" Ryan Good popped out of nowhere in wigs and makeup -- and were quickly followed by Justin himself, who couldn't resist adding some much needed cowbell to the tune.

Willow's brother, Jaden, also joined in on the action -- and started whipping his hair around his surprised sis on stage.

It doesn't look like Willow's going down without a fight though -- after the show she tweeted "my big brothers Justin and Jaden pranked me on stage tonight...#payback!!!

Check out the video, plus photos from the show above!

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