No details on the child's sex or anticipated due date were immediately known.
The couple will celebrate their first wedding anniversary on June 8.
Pratt, 40, is known for his roles in the sitcom Parks and Recreation, and in the Marvel's Avengers blockbusters. He also starred in Jurassic World and its sequel, and was heard in the animated films, Onward and The Lego Movie.
He has a 7-year-old son Jack with his ex-wife, actress Anna Faris.