The singer announced her performance during a video call with Dutch DJ producer Martin Garrix.
She said "I've always wanted to be part of the fun of Tommorowland, and have been following the amazing innovation, technology and invention that this festival has always been about for a long time. I'm glad to be part of this version, and I hope my set makes you smile."
The festival is set to take place between July 25th and 26th and Katy is set to perform as one of more than 60 of the world's most prominent artists.
Other Artists include: Martin Garrix, David Guetta and Afrojack.