Guarini's wife, Reina Capodici, gave birth to son William Neko Bell at 6:26 a.m. William weighed in at 7 lbs., 2 oz. and joins older sister Lola, 5, from Capodici's prior relationship.
Guarini, 32, just wrapped his almost two-month run in Broadway's American Idiot on Sunday.
"Quite a polite fellow for waiting until late Monday, early Tuesday to make his entrance," Guarini told People. "With a full head of curly brown hair, wide open and alert eyes he came charging into the world."