Katy Perry And Keenan Cahill Perform 'Teenage Dream' Duet

Katy Perry has performed a duet with internet sensation Keenan Cahill.

The 16-year-old, from Illinois, has become a hit on YouTube after he began posting videos of himself lip-synching over chart songs.

In September, Perry told Cahill “I heart you” after he released a cover of 'Teenage Dream', which has since gone on to earn 40 million YouTube views.

She returned the favour last week, inviting Cahill to perform the song with her a Well Fargo Center in Philadelphia - you can watch the video on Gigwise below.

The teen suffers from Maroteaux-Lamy syndrome, which limits his growth.

Katy Perry and Keenan Cahill perform 'Teenage Dream:

Katy Perry - live photos