Zoe and husband Rodger Berman made the announcement on Twitter on Sunday.
"So excited to welcome our baby boy into the world … he's 7 lbs., 12 oz., beautiful, healthy and we couldn't be happier," the 42-year-old tweeted.
The designer and Berman, who married in 1996, are parents to son Skyler Morrison, 2.
Rumors that the pair were expecting began swirling in June, but the star of Bravo's sartorially savvy series "The Rachel Zoe Project" did not confirm the news until September.
"We are beyond excited for Skyler to have a sibling and for us to fall in love all over again," she then wrote in a blog post. "We feel incredibly blessed."
Since then, the stylist has been keeping her maternity style tres chic and was even spotted rocking sky-high heels in her ninth month of pregnancy.
Bananas, right?! Well, not quite. The baby bump-concealing expert explained that she doesn't do maternity clothes.
"I don't really have maternity clothes," the stylist told InStyle.com in October. "I don't dress that much differently pregnant than I do when I am not pregnant, honestly. I live in kaftans, ponchos and drape-y tunics because I like to be comfortable, so dressing is not that different. It is the same thing but it takes longer to get ready, I'll tell you that much!"
[Updated 3:25 p.m. Monday, Dec. 23: Zoe reached out to fans via her blog Monday to introduce her second-born in a photo and announce his name: Kaius Jagger Berman. "As loyal readers of The Zoe Report, I wanted you to be the first to see a picture of the beautiful new addition to our family, Kaius Jagger Berman, aka 'Kai,'" Zoe wrote. "Rodger, Skyler and I are madly in love and will be happily hibernating for the next couple of weeks. Thank you for all of your kind words, as they mean so much to us."]
Follow Ministry of Gossip @LATcelebs.