'Everybody's really happy,' a spokesman for the star told People magazine.
The pair are already parents to daughter's Gia, 20 months, three-year-old Isabella and Alexia, 11, Luciana's daughter from a previous marriage.
'Really happy': Matt Damon and wife Luciana are expecting their third child, it has been confirmed
'I really used to have no life outside movies,' he said. 'I'd work all day, go to the gym and go to sleep.
'But once Isabella arrived, that was it for me and working out because I wanted to get home to see my daughter before she went to sleep.'
The 39-year-old met Argentine-born barmaid Luciana when he was filming Stuck On You in Miami in 2003.
Close family: Damon and Luciana with daughters Gia and Isabella
Damon found fame in 1997, winning numerous Best Actor nominations, including the Academy Award, for his role in Good Will Hunting.
The Harvard drop-out co-wrote the screenplay with close friend Ben Affleck.
He went on to star in Saving Private Ryan, the Ocean's trilogy and The Talented Mr Ripley.
Damon is currently filming the Coen Brothers remake of the 1969 John Wayne film True Grit.