King Charles and Queen Camilla Make Surprise Appearance on 'American Idol'

King Charles had a lot on his plate this weekend leading up to and following his Coronation, so it made it all the more shocking he popped up on "American Idol."

The King and Queen Camilla joined hosts Lionel Richie and Katy Perry Sunday night as they live streamed from Windsor Castle... right after Lionel and Katy's Royal performance..

Katy and Lionel, both judges on the show, kicked things off by recapping the weekend and the concert ... Lionel coyly said, "What a party, it was unbelievable. Now we are trying to figure out what we can do to bring something different to the show. So, I have a surprise."

Charles and Camilla both popped up on stage ... with huge smiles on their faces.

Charles said, "I just wanted to check how long you will be using this room for. I just wanted to check." Lionel's response ... "We have to give up the room up right away."

The appearance comes after Charles and Camilla watched LR and KP perform their hit songs from the royal box during the star-studded Coronation concert, which wrapped up the big weekend.

Of course, the events were centered around Saturday's Coronation ... watched by millions around the world.