"Welcome Jaxon Wyatt Cutler 7lbs 11oz 5/7/14," she shared on Instagram.
Baby Jaxon joins the couple's son Camden Jack, 1.
Cavallari, 27, and Cutler, 31, wed in June 2013, after first calling off their engagement in April 2011.
In March, Cavallari made headlines when she revealed her choice not to vaccinate her son Camden -- and presumably now Jaxon -- during a Fox Business interview. She later defended her stance during her appearance on Bravo's Watch What Happens Live! with Andy Cohen.
"There are very scary statistics out there regarding what is in vaccines and what they cause -- asthma, allergies, ear infections, all kinds of things. And we feel like we're making the best decision for our kids," she said. "Here's the thing: At the end of the day, I'm just a mom. I'm trying to make the best decision for my kid."