The 34-year-old pop star surprised fans when she walked on stage dressed as "The Little Mermaid" character Ursula. She dyed her hair bright white, wore bright blue eyeshadow, red lipstick and the iconic shell necklace.
While wearing a floor-length tentacle-covered black gown, Perry explained her reasoning for taking on the persona of The Sea Witch.
"I was thinking about [being] the Little Mermaid but then I realized, Ursula is a character who isn't done very much, [while] the Little Mermaid always gets her moment in the sun! And Ursula, actually, really wants Ariel's voice. And that's what I want, is these contestants' voices!" Perry told Entertainment Tonight after the show wrapped up.
Perry teased her outfit on social media ahead of the episode, in which contestants each took turns singing a Disney-related song, hoping to secure a spot in the Top 8.
"Tonight there will be some poor unfortunate souls Tune in to @AmericanIdol now and VOTE!" Perry tweeted, along with photos of her in the getup. In an earlier tweet, she joked that she was 'ready for my closeup" and "all I want is their voice."
Unlike Perry, Luke Bryan and Lionel Richie kept it casual — sporting black pants, black T-shirts and jackets.
But they appreciated Perry's dedication to the theme night, particularly how she stayed in character throughout the live show. At one point, she even snacked on shrimp cocktail.
"She's the queen of shock factor. I was in my trailer, looking like me, and somebody goes, 'Katy is gonna be Ursula.' And I go, 'Is she purple all over?' And they said, 'She's purple all over.' I went, 'God, I love that woman. I love her,'" Bryan gushed to Entertainment Tonight.
During the episode, other iconic Disney characters joined the judges including Mickey and Minnie Mouse, Donald Duck and Goofy.
Fans praised Perry's dedication to the theme on social media Sunday night.
"YOU REALLY LOOKS SO GOOD KURSULAA. Queen ily," one Twitter user commented.
"Being a little bad," a second wrote.
"YOU KILLED THE DISNEY NIGHT SO HARD THIS TIME YAAAAASSSS QUEEN," another added.