Gellar, 35, married 36-year-old actor Freddie Prinze Jr., in September 2002. They are the parents of a 2-year-old daughter named Charlotte.
"She and Freddie are thrilled," the source said of the baby news. "They're amazing parents."
The child's gender and anticipated birth date were not reported.
Gellar is best known for her work in TV's "Buffy the Vampire Slayer" and "Ringer." She and Prinze co-starred together in the films "I Know What You Did Last Summer," "Scooby-Doo" and "Scooby-Doo 2: Monsters Unleashed."