Katy Perry: Surprise Performer at John Mayer's Brooklyn Concert!

Katy Perry dons a beautiful patterned dress while performing with boyfriend John Mayer held at Barclays Center on Tuesday (December 17) in New York City.

The 29-year-old singer made a surprise appearance to sing “Who You Love,” which is the couple’s new single. Check the video below!

PHOTOS: Check out the latest pics of Katy Perry

Earlier in the day, Katy was casual chic while grabbing a bite to eat with gal pal Allison Williams.

The day before, Katy was camera shy while arriving at JFK Airport in New York City.

FYI: Katy is wearing a Temperley London dress.