The 29-year-old American singer chatted and posed for photos with the 30-year-old English royal at her WellChild performance with Tony Bennett, which raised over $153,000 for the organization.
"We are so happy to be here tonight and be of any help," the pop star told Prince Harry. "We think you are fantastic. And thank you for bringing it here so we can support you."
Prince Harry became WellChild's royal patron in 2007. The charity provides support for U.K. families with seriously ill children, funds research into childhood illnesses and develops children's units at local hospitals.
"WellChild is thrilled that Lady Gaga and Tony Bennett are supporting WellChild through this concert," CEO Colin Dyer said in a statement. "The proceeds will be a massive boost to our work to support seriously ill children and their families across the U.K."
Lady Gaga and Bennett are currently on tour to promote their 2014 album, Cheek to Cheek. The pair will perform Tue. in London before returning to the U.S. next week, and complete the tour Aug. 1 at Kennedy Center Concert Hall in Washington, D.C.