Princess Eugenie and her husband Jack Brooksbank are expecting their second child.
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, 32, also announced the news on Instagram to a flow of congratulatory comments.
She shared a picture of her son August kissing her bump and wrote: "We're so excited to share that there will be a new addition to our family this summer."
The post attracted over 70,000 likes in half an hour and Eugenie's brother-in-law, Edoardo Mapelli Mozzi, commented a couple of heart emojis.
Eugenie and her husband had their first child in February 2021 after marrying at Windsor Castle in October 2018.
The new arrival will be Prince Andrew's and the Duchess of York's third grandchild, after Princess Beatrice's daughter, Sienna, was born in September 2021.
The baby will be 13th in line to the throne.