"From now on, the best part of me will always be my wife and my son", the Full House star captioned the heartwarming pic, in which his son's tiny hand is holding on to his ring finger.
The little boy is named Billy, Stamos said, "after my father".
The new dad announced the pregnancy in a People magazine cover story in December.
John and Caitlin became engaged in October, with the 54-year-old actor sharing the news with a sweet drawing on Instagram of the couple standing in front of Cinderella's castle in Disneyland.
"We have the same morals and the same values, that all clicked nicely", Stamos told People at the time.
"From now on, the best part of me will always be you".
"I asked.she said yes!.And we lived happily ever after", the actor posted.
We honestly couldn't be happier for John and Caitlin! And then you wake up and go, 'No, man, this is it. I've been practising for a long time.