The actress named the newborn, who was 7lbs and 19 inches long, Milo Thomas.
It is the first child for the star and her husband, Hollywood agent David Bugliari.
She quickly shared her joy on Twitter, saying today: ‘Thank you for all the well wishes for my son Milo.
‘My heart has tripled in size. I love him more than all the leaves on all the trees.’
She tweeted: ‘Guys, I’m not over due… Not even close. I’m normal amounts of pregnant. Patience. Let the little dude do what he needs to do. Patience.
She also messaged her former Charmed co-star Rose McGowan with a similar sentiment, writing: ‘He isn’t baked yet! He just emailed me and told me he needs some more time.
In fact, she wasn’t expected to give birth until mid-September.
The actress celebrated the upcoming arrival of her child with a baseball-themed baby shower at Dodger Stadium on August 14.Former child star Alyssa rose to fame in the Eighties sitcom Who’s The Boss? and more recently played the character of Phoebe Halliwell on the supernatural series Charmed.