Pentatonix surprise Katy Perry with impromptu performance

Three members of acapella group Pentatonix gave Katy Perry a big shock and surprise when they turned up singing her hit song 'Chained to the Rhythm'.

The group snuck in as Katy Perry was getting her make up done for her concert, leaving Katy wide eyed and smiling as her live stream recorded the moment.

She couldn't help but join in on the performance too.

Watch Katy and Pentatonix sing together in the video above.