The former "Buffy the Vampire Slayer" star married actor Freddie Prinze Jr. in 2002. They have a 2-year-old daughter, Charlotte Grace.
"They are thrilled that Charlotte will have a little brother," a source close to the couple told People.com. "They love their little girl more than anything in the world and know that love will only multiply."
Gellar and Prinze co-starred together in the films "I Know What You Did Last Summer," "Scooby-Doo" and "Scooby-Doo 2: Monsters Unleashed."
They have not revealed the anticipated due date of their second child. Gellar's pregnancy was first reported in April.