The couple celebrated Independence Day by taking to Twitter to announce the arrival of their daughter, Violet.
"What better way to celebrate the 4th... than to announce our 4th family member!!!" wrote Krasinski, 36. "2 weeks ago we met our beautiful daughter Violet #Happy4th."
The pair, who will celebrate their sixth wedding anniversary on July 10th, are already parents to two-year-old daughter Hazel.
Blunt, 33, recently revealed that little Hazel was still getting her head around welcoming a sibling.
"She's, like, perplexed, you know?" the Huntsman: Winter's War star told ET in an exclusive interview. "How on earth are you supposed to understand that there's a baby in there? It makes no sense to her, I'm sure."
- Jan. 26, 2016 Emily Blunt and John Krasinski are having another baby