The 47-year-old actor's daughter, Jane Carrey, and her husband, Max Santana, welcomed son Jackson Riley Santana on Feb. 26 in Los Angeles.
The baby weighed in at 7 lbs, 11oz and measured 20 inches long, according to People.
Carrey tweeted about the arrival, posting a picture of the baby and writing, "Jackson Riley Santana, My Grandson, Born today! This is what 7lbs.11oz. of Calfornia dynomite looks like!"
Jane Carrey, 22, is the lead singer in the Jane Carrey Band, while her husband is the lead singer of the metal band Blood Money.