The 34-year-old singer had been seen enjoying the entertainment at the popular festival in Indio, California with fiance Orlando Bloom earlier in the weekend, but had not been scheduled to perform.
So onlookers watching DJ Zedd on the decks on Sunday could be forgiven for their surprise when Katy joined him to perform their new collaboration "365".
Wearing a red tie-dyed jumpsuit with a diamond net cover-up over the top, Katy sparkled as she danced around the stage during the performance, before telling the crowd in the middle of the song: "I used to sneak in here!".
Later on, she added: "I've been doing Coachella for about 17 years. Let me see you move. Will you move for me cos I'm a fan for you?".
The crowd happily obliged, much to the delight of the "American Idol" judge.
Zedd and Katy's new song was announced earlier this year, with the DJ saying at the time: "Katy and I have known each other for a while now and while on tour together in Australia last summer, we started working on some music. After we got off tour and with our musical value systems even more aligned we continued working together and that's when 365 started to take shape. As it all came together, we knew we had something special. I love how Katy sounds on this record."