McConaughey made the announcement on his Web site.
"Got some blessed news ... a celebration of life and bounty ... a newborn concieved (sic) ... yes, my girlfriend Camila and I made a baby together," McConaughey wrote.
Alves, who is three months along, is doing well, the actor wrote.
"We are stoked and wowed by this miracle of creation and this gift from God, and so excited for the adventure that will come in raising this child, being a mother and a father," he said.
McConaughey goes on to say, "Wish us the best, keep us in your prayers, and God bless evolution."
McConaughey, a Texas native, has appeared in films including "The Wedding Planner" and "Sahara" and was named People magazine's "Sexiest Man Alive" in 2005.
His latest film, "Fool's Gold," is to be released next month.
