The 27-year-old actor's wife, Elizabeth Chambers, is pregnant with their first child, which is due later this year.
A rep for the star confirmed the happy baby news to People but declined to reveal any further details.
Los Angeles-born Armie appeared in various TV shows including Gossip Girl and Desperate Housewives before finding fame in The Social Network, the movie based on the creation of Facebook.
Since then, he has appeared in movies including J. Edgar, Mirror Mirrorand The Lone Ranger, alongside Johnny Depp.
Archie married model and TV host Elizabeth in 2010.
Speaking about their courtship to Town & Country recently, Elizabeth said: "When we finally got together, Armie really wanted to get married.
"He sat me down and said, 'Look, we don't have to do this. We could just go our own ways, and then one day you'll be 40 and divorced and we'll run into each other, and we'll laugh and go out to dinner and have this same connection, and we'll wonder why we wasted all of that time. Or we could just do it now and enjoy the ride.'"
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