The 54-year-old Stamos posted a photo his son's arrival to Instagram, saying, "From now on, the best part of me will always be my wife and my son. Welcome Billy Stamos."
He tagged it: "#NotJustanUncleAnymore #Overjoyed."
Billy is named after John's father.
In December, the Full House and Fuller House star announced that he and fiancee Caitlin McHugh were expecting a child in an interview with People Magazine.
Stamos, who is a first-time father, said he was ready because he's had "years of practice."
----------
* More local news
* Send us a news tip
* Download the abc7NY app for breaking news alerts