"Immensely proud of you my my love," the actor captioned a May 8 post featuring photos from the big night. "You always bring the light."
The excitement from Katy's fiancé left her roaring with glee, as she wrote in the comments, "Our light" with a heart emoji.
The "California Gurls" singer took the stage at Windsor Castle on May 7 for a concert held in honor of King Charles III and Queen Camilla's coronation. During the show, Katy wore a gold Vivienne Westwood gown and performed her hits "Roar" and "Firework."
"I'm so happy to celebrate this whole weekend," the American Idol judge told the audience. "I got to bring my mom. She is so happy to be here. We get to stay in Windsor Castle. No big deal."
Katy—who was appointed ambassador to the British Asian Trust by Charles in 2020—also expressed her gratitude to His Majesty for inviting her to the historic occasion.
"Thank you for having me at this coronation celebration," she added. "I'd like to dedicate this next song to the King and to the work that we get to do together for the British Asian Trust and their children's protection fund. Thank you for bringing out the firework in so many young people."
Katy was part of a star-studded lineup that also included performances by Lionel Richie, Andrea Bocelli and Nicole Scherzinger as well as pre-recorded messages from Hugh Jackman, Joan Collins, Pierce Brosnan and Tom Cruise. In addition, she got to attend the coronation ceremony at Westminster Abbey in London on May 6. Although, she initially had some trouble determining where she was supposed to sit.
"Don't worry guys," Katy—who wore a lilac suit by the same designer—tweeted after footage of her searching went viral, "I found my seat."
To see photos from the coronation concert, keep reading.