The princess - who is 11th in line - shared the news on Instagram, where she posted a touching image of August kissing her tiny bump.
Alongside the photo, which was taken by Jack, Eugeine wrote: We’re so excited to share that there will be a new addition to our family this summer.”
In a statement, Buckingham Palace said: “Princess Eugenie and Mr Jack Brooksbank are pleased to announce they are expecting their second child this summer.
“The family are delighted and August is very much looking forward to being a big brother.”
Eugenie and Jack welcomed August on February 9, 2021, at Portland Hospital in London.
He was delivered by caesearean section because Eugenie had a scoliosis operation on her spine as a child. August weight 8lbs 1oz.
The couple met in 2011 and their first outing together was at Royal Ascot that year.
They went long distance in 2013 when Eugenie moved to New York to work in an auction house, but stayed in touch on Skype.
She moved back to London in 2015, but brand ambassador Jack waited until 2018 to pop the question while they were on holiday in Nicaragua.
They tied the knot at St George’s Chapel in Windsor in 2018.
They now sublet Prince Harry and Meghan Markle’s Frogmore Cottage, where they stay when in England, but spend some of their time at the CostaTerra Golf and Ocean Club in Portugal, not far from where Jack works.
Eugenie’s older sister, Beatrice, has one child, daughter Sienna, with her husband Edoardo Mapelli Mozzi.
Eugenie spoke earlier at the World Economic Forum in Davos about how becoming a mother had “totally changed” her outlook on the environment.
“My son’s going to be an activist from two years old, which is in a couple of days,” she said. “Everything is for them, right? Every decision we now make has to be about how August is going to be able to live his life.
“But I think also as a mother, you all of a sudden, totally you change, your hormones change, everything changes.
“Like now I’m scared of flying and things like that and I would never be before.”
Eugenie added that she had made changes at home so her family were more green.
“At home we have no plastic, we try to as much as possible have no plastic and I’m trying to teach him that. But it’s a battle.”
She later said: “Modern slavery and human trafficking is a really big issue across the globe.
‘There are 49 million people estimated in slavery today and we know that when the climate is vulnerable, the most vulnerable people are affected by it.
“And we’re going to see that more and more, you know, each time there’s a crisis happening, that people are going to be vulnerable and taken into difficult situations.
“So climate goes hand in hand with it really.”
