Have mercy! Baby Stamos is here!

John Stamos is officially a father and his adorable announcement has us saying, "Have mercy!"

Stamos, 54, 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."