"Wanted to let the news out directly," Krasinski, 34, said early Sunday on Twitter. "Emily and I are so incredibly happy to welcome our daughter Hazel into the world today! Happy bday!"
It's a first child for Blunt and her "The Office" star hubby, who got married in 2010 in Italy. They announced their engagement in August 2009 after dating since November 2008.
They announced the pregnancy in September.
The "Looper" and "The Devil Wears Prada" actress, who turns 31 this week, celebrated the baby in advance at what a People magazine source called a "very casual" shower in December. The fete was reportedly attended by the likes of Jennifer Aniston, Kristen Bell and Amy Adams.
"I feel like everyone poo-poos actor relationships because they say they always end in divorce and tears," Blunt told The Times in April 2012. "But that's only because it's in the news and it's broadcast to the world. John and I can talk about everything, and it's nice, because we have a very deep understanding of what the other person does."