The 63-year-old actress will step into the recurring role of Patty Atwood, the director and choreographer of the Broadway play-within-a-show. Sarandon joins previously announced cast members including Hope Davis, Frank Langella and Law & Order's Linus Roache.
The Miraculous Year follows a self-destructive Broadway composer trying to balance his equally difficult new show and family. The project will be produced by Tony winner John Logan and Oscar winner Kathryn Bigelow, who will also direct the pilot.
Sarandon, an Oscar winner for Dead Man Walking, recently snagged an Emmy nomination for supporting actress in a miniseries or movie for another HBO project —You Don't Know Jack.
Production on the pilot begins this month. Reps for HBO could not immediately be reached for comment.