A Kensington Palace spokesman said William and Kate were “absolutely delighted with the news” of the 32-year-old’s engagement at the weekend, which will be followed by a wedding next year.
Middleton rose to global prominence when she was a bridesmaid at her sister’s wedding in 2011.
She appeared in a figure-hugging white dress by Alexander McQueen, pictures of which filled newspapers for weeks afterwards.
Middleton subsequently wrote a book on party planning that was widely mocked for offering obvious tips, followed by a regular column in a magazine for upmarket British supermarket chain Waitrose.
“Miss Pippa Middleton and Mr James Matthews are delighted to announce they became engaged on Sunday July 17 and plan to marry next year,” they said in a statement.
The couple have been together less than a year but they were living together.
Matthews proposed during a trip to the picturesque Lake District in northwest England, the Daily Mail newspaper said.
Middleton’s parents Michael and Carole said they were “absolutely thrilled”, with her father saying in a statement: “They make a wonderful couple and we wish them every happiness together.”