She performed the iconic title song in the curtain call of last night's (October 25) performance of the hit musical, alongside former Royal Ballet principal dancer Adam Cooper.
The performance was part of the BBC Children in Need charity project POP Goes the Musical, which is giving popstars the opportunity to perform a song at select dates for hit musicals, with 50% of money raised from ticket sales going towards helping disadvantaged children.
Geri said: "Performing in Singin' in the Rain in the West End was an absolute blast and a privilege to be lifted by the gorgeous Adam Cooper. The audience were fantastic, it was a dream come true."
Stars who have taken part so far include Blue, who joined the cast of Jersey Boys for a rendition of 'Who Loves You', and Alexandra Burke, who appeared alongside members of the Les Misérables ensemble to perform 'On My Own'.
Coming up on November 15 is the final POP Goes the Musical performance, which will feature Macy Gray, Stooshe and Joe McElderry 'popping up' at the end of a performance of Thriller: Live.
Tickets cost £80 and are available at Lastminute.com.