Krasinski, 36, tweeted Monday, "What better way to celebrate the 4th...than to announce our 4th family member!!! 2 weeks ago we met our beautiful daughter Violet #Happy4th."
He and Blunt, 33, revealed the pregnancy in January. They are also parents to 2-year-old daughter, Hazel.
Krasinski starred earlier this year in the Michael Bay-directed movie "13 Hours." Blunt was featured most recently in "The Huntsman: Winter's War" and will appear in the adaptation of the best-selling book "The Girl on the Train," opening in October.