The Hollywood couple, who already have two-year-old Hazel, announced the addition of one more to their brood: a second daughter, Violet.

Krasinski, 36, shared the news in a Twitter post on Monday, revealing Blunt, 33, had given birth two weeks ago.

"What better way to celebrate the 4th... than to announce our 4th family member!!! 2 weeks ago we met our beautiful daughter Violet," he tweeted.

Speaking to People at the premiere of her film Into the Woods in 2014, when first daughter Hazel was 10 months old, Blunt described motherhood as "the best thing ever".

"You don't know what to expect because it's just completely, existentially out of anything that you've ever experienced before, and it's quite indescribable."

Blunt and Krasinski started dating in November 2008, and married in Como, Italy, in July 2010.

In a 2011 appearance on The Ellen Show, Krasinski retold the story of how he first met Blunt before he was cast for the US season of The Office.

"It was one of those things where I wasn't really looking for a relationship and I was thinking I'm going to take my time in L.A.," the actor said.

"Then I met her and I was so nervous. I was like, 'Oh god, I think I'm going to fall in love with her.' As I shook her hand I went, 'I like you.'"