Guarini's wife, Reina Capodici, gave birth to the seven lbs., two oz., brown-haired baby boy at 6:26 AM April 26.
The "American Idol" finalist has been seen on Broadway in Women on the Verge of a Nervous Breakdown and performed as Will in the closing cast of American Idiot.
He will star in Rent alongside MTV's Paul Iacono, who will play Mark, at Surflight Theatre in Beach Haven, NJ, May 28-June 19.