The celebrity couple are already the parents of 11-month-old Levi, People.com noted.
"Happy Father's Day. It's my first, and the last 11 months with Levi and Camila have been the most rewarding adventure to date," McConaughey wrote on his Web site Sunday. "We have more blessed news to celebrate this Father's Day that (will) make this time next year double the fun. Levi is going to be a big brother... Yeah, we pulled off the greatest miracle in the world one more time. Camila and I are expecting our second child, bringing more life into the world, making more to live for. The future looks bright as the family grows.... Just keep living, Matthew and Camila."
McConaughey did not reveal the baby's gender or due date.