Gellar's first child with her husband, actor Freddie Prinze Jr., was born Saturday, E! News said.
The couple has been married for seven years.
"The family is over the moon," Gellar's representative told People.com.
Gellar is best known for her work on TV's "Buffy the Vampire Slayer."
She and Prinze have appeared together in the films "I Know What You Did Last Summer" and "Scooby-Doo."