The former Gossip Girl star, 31, made the announcement by showing off her baby bump with husband Ryan Reynolds on the yeloww carpet of Reynolds’ latest movie premiere in New York.
Fittingly, Lively wore a canary yellow-coloured dress for the screening of Pokemon Detective Pikachu in Times Square.
The couple has so far not announced when Lively is due.
Lively and Deadpool star Reynolds — who tied the knot in 2012 — already have two daughters James, 4, and Inez, 2.
Like the pregnant Duchess of Sussex, Lively is an absolute pro at maternity dressing and famously styles herself, often wearing designer pieces by Gucci and Michael Kors that are straight off the runway.
Lively previously told Women’s Wear Daily that she chooses her own outfits because she has “control issues and a big ego”.
“I love design and I love fashion, and it’s a way to be creative,” Lively said, in an interview with WWD last year.
“In my job I get to be creative, but it’s over a period of time and so many other people are involved, whereas this is a beginning, middle, and end, and I get to be creative and there’s an end date in the near future.”
Lively is usually a regular on the red capret at the annual Met Gala, which is being held next week.
Meantime, the couple has spoken about being parents.
“When (birth) happens you fall so much more in love with your partner. It’s a pretty crazy experience,” Reynolds told E! in 2015.
Lively has previously spoken about how she and Reynolds make their relationship work despite their busy Hollywood schedules.
“We’ve never gone a week without seeing each other,” Lively told Marie Claire magazine in 2014, according to E!
“here’s no major decision that I make without him. The best part is when we turn off our phones and just talk and hang out.
“He’s my best, best friend. What do you do with your best friend? You do nothing.”