The 25-year-old singer made a special appearance at annual event to present the best actress award with Jonah Hill to teen favourite Miley Cyrus.
But when she opened the box to reveal the winner, Katy found herself sprayed with the show's signature surprise slime, covering her from head to toe.
Despite tweeting fiancé Russell Brand about the possibility of being gunged earlier that day, Katy looked genuinely surprised at the prank.
That didn’t stop her taking it all in her stride though, and after being drenched she turned to Jonah – Russell's co-star in Forgetting Sarah Marshall – for a big slimy hug.
She joins a prestigious list of celebrities caught off guard at the ceremony, from Sandra Bullock and Hugh Jackman, to Johnny Depp, Nicole Kidman and Harrison Ford.
Among those who were relieved it was not their turn to face the green gunge was Twilight hunk Taylor Lautner.
He was named favourite film actor, and also beat off competition from co-star Robert Pattinson to win cutest movie couple with Kristen Stewart.
"I have to admit, I have always dreamt of winning one of these orange blimps," the 18-year-old confessed. "Seriously."
Other stars who turned out for the event included Rihanna - debuting a new two-tone hair cut - who performed a medley of songs.
And Will Smith was there too, making it a family affair by bringing along his whole clan, including son Jaden, 11, who posed on the orange carpet with his Karate Kid co-star Jackie Chan.
Avatar's Zoe Saldana, Canadian singer Justin Bieber, and the Jonas Brothers, Kevin, Nick and Joe, were also at the event – Kevin arriving with his wife Danielle and Joe stepping out with his new love interest, Demi Lovato.