Pentatonix's surprise performance with Katy Perry

Pentatonix dropped by Katy Perry's Witness World Wide live stream to serenade her.

Three of the groups members - Kirsten, Mitch and Scott - surprised the Roar hit maker while she was having her hair trimmed, performing an a-capella version of one of Katy's latest songs, Chained to the Rhythm.

Katy joined in with Pentatonix's singing half way through the tune and later expressed her excitement at seeing the group, saying "this is wild!"

Watch their performance above.