Cardellini, 36, is due to deliver the baby early next year, her representative told Us Weekly Tuesday, without divulging the child's gender.
The baby will be the first for the actress and her boyfriend Steven Rodriguez.
Cardellini is best known for her work on TV's "Freaks and Geeks" and "ER." She also appeared in the recent live-action Scooby-Doo movies and the film "Return."
Swim Week Miami Beach 2014 [PHOTOS]