Created by Oscar winner Diablo Cody, "Tara" focuses on a wife and mother (Toni Collette), whose dissociative identity disorder makes things hilariously complicated for her family, including her husband (John Corbett).
Larson will play the main character's rebellious daughter.
According to The Hollywood Reporter, Portia Doubleday played the role in the original pilot, but was recast.
Showtime recently gave a 12-episode order to the DreamWorks TV production.
Credits for Larson include "Sleepover," "Hoot" and the upcoming "Peck."