Former reality star Cavallari, 28, shared the news on Instagram Monday night, introducing her new daughter as Saylor James Cutler. She was born at 1:32 that afternoon.
The celebrity couple are also parents to sons Jaxon, 18 months, and Camden Jack, 3 years.
The Laguna Beach star and fashion designer announced she was pregnant in May of this year, writing in a public message: "We're at it again! Thrilled to announce baby number 3 is on the way! The Cutler crew keeps on growin'!" Two months later, she revealed the sex of her new baby over social media.