The Parks and Recreation star, 38, gave birth to the couple's second son Friday morning, her rep tells PEOPLE.
Abel James Arnett weighed in at 7 lbs., 13 oz., and joins older brother Archie, who was born in October 2008.
"Amy, Will, Abel and Archie are all healthy and resting comfortably," says the rep.
Parenthood, Arnett, 40, has raved, is "just the best."
"It's a human being that you're responsible for," he said in January. "That's scary, but even in its scariest moments it's fantastic."
Reporting by MARISA LAUDADIO