The couple announced that they were expecting their second child in April. The pair already have 2-year-old daughter Charlotte together.
"They are thrilled that Charlotte will have a little brother," a source told People.
"They love their little girl more than anything in the world and know that love will only multiply," the insider continued.
Gellar's TV series Ringer was recently cancelled by The CW after only one season.
The actress previously claimed that she and Prinze Jr fight over who gets to stay home with their daughter.
The pair have been married since 2002.