The actor says on his Web site that Vida Alves McConaughey was born Sunday. She weighs seven pounds, seven ounces.
McConaughey says his girlfriend, model Camila Alves is "recovering wonderfully" and the couple welcomed "this little lady into our family."
The couple had a son, Levi, in 2008.
McConaughey spokesman Alan Nierob confirmed the Web posting in an e-mail on Monday.