The baby joins big sisters Violet, 6, and Seraphina, 3.
The couple, both 39, announced the pregnancy in August. In January, Garner told Jay Leno that they knew the sex of the baby, but were keeping it a secret.
"It would be so weird to have a boy," Garner told Ellen DeGeneres, but also said the experience would be "cool and different."
UPDATE: The proud father has just shared the happy news via his Facebook account. And the baby's name is quite traditional -- refreshing in the land of celebrity.
We are happy to announce on February 27, Jennifer gave birth to a healthy baby boy, Samuel Garner Affleck.
Congratulations to the happy family!