The "Girls" star, who nabbed a Tony Award nomination for his role in "The Book of Mormon," is set to take over for Neil Patrick Harris, whose final performance in the musical will be Aug. 17.
Rannell's first performance in the Tony-winning production will be Wednesday, Aug. 20.
As it turns out, it isn't Rannells' first time playing the character. As he tells Fallon, he previously starred as Hedwig in a 2002 production in Austin, Texas.
Meanwhile, Harris sounded off on his impending departure from the show on Instagram: