The 'Teenage Dream' singer went for an uber dramatic and glamorous look as she supported her designer friend Jeremy Scott at the premiere of film Jeremy Scott: The People's Designer on Tuesday evening (8 September).
Katy looked sensational in a floor-sweeping black evening gown and voluminous, back-combed hair. All very Angelina Jolie, right?
She's worked with designer Jeremy for years and is one of his biggest fans: he designed all her outfits for her Super Bowl performance earlier this year.
He told Elle: "Katy called me in October, when I was in Milan for the Moschino show. I knew something was up, because she's like, 'Are you home? Can we get together? Because we need to chat.' Then [her stylist Johnny Wujek called me, and said, 'I need to talk to you.'
"I knew something major was happening, but I couldn't figure out what. I was like, 'Is she getting married? No. So what's going on. A week later, we got together. She said, 'I've accepted the Super Bowl halftime slot. It hasn't been announced yet, but it's a very important moment for me, so it needs to be an amazing moment. But I want it to be amazing for Katy Perry and Jeremy Scott.'
"It was very generous of her to say that, because this really is her moment. She can take this to the bank!"
Jeremy has also worked with stars like Miley Cyrus and Rita Ora. The film will show fans what it's like behind the scenes as they prepare for events like the Super Bowl or the red carpet.
He told WWD: "There's a lot of people who know the iconic image of Katy Perry at the Super Bowl or Miley and me after my first Moschino show, but in this film, you see Katy and I in our car on the way to the Met and in the locker room bathroom before she goes on to the Super Bowl field. You get these behind-the-scene glimpses of what happened before all these moments."