The Life Unexpected actor will play the male lead character Sebastian Kydd in the Sex and the City prequel.
Based on the book of the same name by Candace Bushnell, The Carrie Diaries follows a teenage Carrie Bradshaw attending high school in the 1980s.
Sebastian, who goes to the same school as Carrie, is described as a "brooding heartthrob and ultimate insider" by Deadline.
AnnaSophia Robb previously signed on to play the role of Carrie, while Katie Findlay and Ellen Wong have also been added to the cast.
The Carrie Diaries is being developed by The O.C. and Gossip Girl showrunners Josh Schwartz and Stephanie Savage.
Butler has had guest spots on CSI: NY and Are You There, Chelsea?, and appeared in a recurring capacity on Switched at Birth.