Vanessa announced the good news on Instagram.
"It's a very Merry Christmas indeed! We had our Christmas Miracle. Phoenix Robert Lachey decided to show up early and was born on Christmas Eve. Mommy, Daddy, Camden & Brooklyn LOVE You very much! #LacheyPartyOf5," she wrote next to a photo of the baby's hand on top of hers.
The couple first announced they were expecting in September and that the baby was due in spring 2017.
The little one joins sister Brooklyn Elisabeth, nearly two, and brother Camden John, who is now four.