Princess Eugenie and Jack already share a son, August, who will be turning two years old on February 9.
Their first son was safely delivered in London's Portland Hospital, with Buckingham Palace confirming the news.
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The royal announced the news of their second baby on Instagram early Wednesday morning (Australian time), sharing a photo of her first son August kissing her stomach.
"We're so excited to share that there will be a new addition to our family this summer," she wrote.
Buckingham Palace also released a statement, which read: "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," the statement ended.