Publicist Leslie Sloane says the baby was born in Los Angeles. His name and details of the birth weren't released.
The couple also has a 3-year-old daughter named Charlotte. They celebrated their 10th wedding anniversary this month.
Gellar and Prinze have co-starred in two "Scooby-Doo" films along with "I Know What You Did Last Summer."
Last year, the 35-year-old Gellar starred in the CW drama "Ringer." The 36-year-old Prinze had a role on the final season of Fox's "24."
Us Weekly first reported the birth.