Princess Eugenie and her husband Jack Brooksbank are expecting their second child.
Eugenie, 32, announced the news with a photograph of her son, August, kissing her bump.
"We're so excited to share that there will be a new addition to our family this summer," she wrote on Instagram.
Buckingham Palace said the Royal family is "delighted" and that August is "very much" looking forward to becoming a big brother.
The baby will be born 13th in line to the throne and is not expected to receive a royal title.
He or she will be the first royal child to be born since the death of Queen Elizabeth, and will arrive in the King's coronation year.
Eugenie gave birth to August at the Portland Hospital in central London in Feb 2021.
The family first revealed him to the world wrapped in a blue blanket and a matching knitted hat.
He made a further appearance at the Platinum Jubilee pageant celebrations last year wearing a union flag jumper.
Eugenie revealed at the World Economic Forum in Davos last week that August will be "an activist from two-years-old".
She added that becoming a mother had made her more concerned about climate change and that all her decisions now revolve around her son.
"Every decision we now make has to be for August, what he's going to be able to look at and do and how he's going to live his life," she said.
"But I think also as a mother, you all of a sudden totally change, your hormones change, everything changes.
"Like now I'm scared of flying and things like that and I would never be before."
The couple recently moved to Portugal after Mr Brooksbank, 36, started a new job with Mike Meldman, a developer of luxury golf resorts.
They will now divide their time between the CostaTerra Golf and Ocean Club resort and the UK.