Eugenie married Brooksbank at Windsor Castle in 2018 and their first child, a son named August, was born in February 2021.
"Princess Eugenie and Mr Jack Brooksbank are pleased to announce they are expecting their second child this summer," Buckingham Palace said in a statement.
"The family are delighted and August is very much looking forward to being a big brother."
Eugenie, 11th in line to the British throne, is the younger daughter of the king's brother Prince Andrew and his ex-wife Sarah Ferguson, the Duchess of York.
(Reporting by Kylie MacLellan; editing by Michael Holden)