Milo Thomas Bugliari is Milano's first child with husband David Bugliari, who she married in August 2009.
She tweeted Wednesday that her heart "has tripled in size" and that she loves her new son "more than all the leaves on all the trees."
Milano starred in TV's "Who's the Boss?" and "Charmed" and will next have a role in the ensemble comedy "New Year's Eve." Her husband is an agent with Creative Artists Agency.
The Associated Press contributed to this story.