Geri's cameo was all in aid of the BBC's annual 'Pop Goes the Musical' fundraiser which sees famous faces making one-off appearances in London shows to help raise cash for Children In Need.
The former Ginger Spice donned a suit and trilby for the show's big finale where she sang and danced her way through a very wet version of Singin' In The Rain, orginally made famous by Gene Kelly, Debbie Reynolds and Donald O'Connor in the 1952 movie.
Geri was all smiles as she left the Palace Theatre where she signed autographs at the stage door.
She later took to Twitter to thank her fans for their support, writing: "Thank you so much for tonight, everyone at the theatre were gorgeous, so lovely to see my fans. You're the best in the world, so loyal!"