Galifianakis, who has kept quiet with information about his son, confirmed that the little one is about a year old, and joked when asked about the child's gender, "We don't wanna find out until it's 16. We just call it 'it' right now."
The Birdman or The Unexpected Virtue of Ignorance star, 45 — who made headlines this week for his dramatic weight loss — welcomed his first child with wife Quinn Lundberg in September 2013. The couple tied the knot in August 2012.
"We haven't named it yet," Galifianakis joked to Fallon of his child. "It has a number right now: one. The first one!"
He then paused his humorous antics to confirm that his tot is a son. "We have a boy," the actor said.
"I think fatherhood is just the greatest thing," Galifianakis gushed. "It really, really is."
Watch Galifianakis talk being a parent and bathing with his son in the video above!