Allison also showed the picture on her brand new Instagram account.
She captioned the shot: "Thrilled to be playing Peter Pan live on NBC in December. I've been rehearsing for a REALLY long time."
In an official statement she also added: "
I have wanted to play Peter Pan since I was about 3 years old, so this is a dream come true."
Allison Williams with her Girls co-stars
Peter Pan Live will air on the NBC network in December. It follows the success of the live production of The Sound of Music which starred Carrie Underwood in the role of Maria, originated by Julie Andrews in the 1965 film.
Christopher Walken will star as Hook.