"Camila Alves gave birth to our third child yesterday morning. Our son, Livingston Alves McConaughey, was born at 7:43 a.m on Dec. 28, 2012," McConaughey wrote. "He greeted the world at 9 lbs. and 21 inches. Bless up and thank you for your well wishes."
The couple's new arrival joins older siblings Levi, 4, and Vida, 3.
McConaughey, 43, announced his wife's pregnancy via Twitter July 4, just weeks after the couple tied the knot in Austin, Texas. Together since 2006, the couple swapped vows in front of several A-list pals, including Reese Witherpsoon and Woody Harrelson.
Several months into her third pregnancy, Brazilian model and handbag designer Alves, 30, told Us Weekly her two children were eager to welcome another sibling. "I think they fully understand that it's a little baby coming," she said in September. "They love it."