E! News said Schwimmer posted a photo of himself with Buckman, a British photographer, and their newborn Sunday.
The actor's spokeswoman said Cleo was born May 8.
The couple, who have been together for about four years, married last June.
Schwimmer is best known for his work on the sitcom "Friends." He also directed the film "Run Fat Boy Run" in England.
Swim Week Miami Beach 2014 [PHOTOS]