The baby boy joins the couple's daughter Vida, three, and their four-year-old son Levi.
Matthew shared the news with the world when he took to his WhoSay account and wrote: "Our son, Livingston Alves McConaughey, was born at 7.43am on 12.28.12. He greeted the world at 9lbs and 21 inches. Bless up and thank you for all the well wishes. Happy New Year and just keep livin."
It is believed 30-year-old model Camila gave birth in Austin, Texas, where the two share a home, and where they married in June after a six year romance.
Actor Matthew, 42, is currently a shadow of his former buff self after losing a reported 40lbs for his role in the film The Dallas Buyers Club.
He plays the role of Ron Woodruff, who smuggled life-saving medicine into the United States, despite being diagnosed with AIDS.
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