Armstrong, 38, will play the role of St. Jimmy in the Tony Award-winning Broadway play starting on Tuesday.
Tony Vincent normally plays the part, but Armstrong said he would step in to take his place after Vincent said he needed “a leave of absence due to a personal family matter," according to Entertainment Weekly.
The role shouldn't be a problem for Armstrong as it entails singing many Green Day songs.
"American Idiot" tells the story of three young men's lives in the post 9/11 world. Armstrong will sing "Know Your Enemy," "Homecoming" and "St. Jimmy," all taken from the album of the same name.
Armstrong is slated to return with Green Day on Oct. 8.
