A source tells ET the Magic Mike star was present for the baby's birth Friday at a hospital in Austin, TX.
On Saturday, he shared the happy details with the world, writing on Whosay: "Camila (@iamcamilaalves) gave birth to our third child yesterday morning. 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 your well wishes. Happy New year, and just keep livin, Camila and Matthew McConaughey."
McConaughey proposed to his 30-year-old Brazilian model/handbag designer girlfriend on Christmas Day 2011, and they got married during a ceremony in Texas last June.
The couple began dating in 2006 and have two other children together, daughter Vida, who is almost three, and four-year old Levi.
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