Both were flaunting their assets as they posed together during the event which raises money for the Foundation's AIDS research programmes.
Kate took the plunge in a daring red dress, while Katy (whose real surname is also Hudson, fact fans) put on more than a show as she took to the stage to perform at the event.
But Katy's charitable turn didn't end at treating guests with the sight of her famous cleavage, as the singer is set to donate one of her guitars to help raise money for the organisation.
A spokesperson for the charity said: "We're thrilled to announce that superstar Katy Perry has donated a guitar to auction off at amfAR's Inspiration Gala Los Angeles tomorrow. Katy will personalise an autograph on the guitar to the lucky winner. All proceeds benefit our research programs... Thank you, Katy!"