Vanessa Lachey took to Instagram to announce the exciting arrival, posting "It's a very Merry Christmas indeed!"
"We had our Christmas miracle. Phoenix Robert Lachey decided to show up early and was boring on Christmas Eve," she continued.
Phoenix is a brother for the couple's four-year-old son Camden John and soon-to-be two-year-old daughter Brooklyn Elizabeth.
"Mummy, Daddy, Camden & Brooklyn LOVE You very much," the 36-year-old exclaimed.
It was only in September that the couple announced that they were expecting after they shared a photo of the family in front of their new home, with Vanessa showing off her baby belly.