The singer said that the group hope to premiere the highly-anticipated musical, called Viva Forever, "towards the end of next year", and praised its "fantastic" producers Judy Kramer and Jennifer Saunders.
"It's sounding really exciting. There's lots of great Spice Girls songs in the show, lots of hits, some album tracks, so we're very excited," she told Digital Spy.
"Jennifer Saunders has written the story, who as everybody knows is hilarious and brilliant. We've known Jennifer for many years - she was in the Spice Girls movie!"
Asked if the musical could spark another Spice Girls reunion tour, she admitted: "Who knows, we don't plan to get together and perform again. I didn't think it would happen the first time."
Melanie C releases her new single 'Think About It' on September 4 and album The Sea a day later.
Watch Melanie C discuss the Spice Girls musical below: