The Magic Mike actor and Alves revealed the pregnancy in near-identical messages on their respective Twitter pages on Wednesday (July 4).
"Happy birthday America, more good news, Camila and I are expecting our 3rd child," McConaughey wrote. "God bless, just keep livin."
The couple married in a private Texas ceremony in June, after more than six years together.
They are already parents to 4-year-old son Levi and 2-year-old daughter Vida.
McConaughey has previously said that his wife is perfect for him and that he is fully committed to their partnership.