"I've gotten to spend time with her and I know how deeply she cares about making our world a better place," Clinton said. "We need champions like Katy now more than ever."
Clinton, whose appearance surprised both Perry and the crowd, said that the singer "has the spirit and the energy and the compassion that Audrey Hepburn brought to her work for UNICEF."
Perry has been an ambassador for the organization since 2013, working to end child poverty and advocating for the empowerment of girls and women across the globe.
Perry, who also supported Clinton during her presidential run, performing at the Democratic National Convention and several Get Out the Vote concerts, thanked the former Secretary of State in her acceptance speech.
"You motivate me and so many millions of people," Perry told Clinton.
Also honored at Tuesday's event was interior designer Moll Anderson.