A statement added: "Everyone is delighted and mother and baby are doing well".
Pippa was dubbed "her royal hotness" after wearing a slinky, white dress to older sister Catherine's royal wedding in 2011 where she was a bridesmaid.
The baby was born on the day the palace announced that Prince Harry and his wife Meghan, the Duchess of Sussex, are expecting their first child.
The new cousin for Prince George, Princess Charlotte, and Prince Louis was born at 1.58 pm and weighed 8 pounds and 9 ounces. A suite, where Kate is believed to have given birth, is thought to cost closer to £7,500.
The news comes just days after Middleton and Matthews, who Wednesday in May 2017, were photographed attending the October 12 wedding of Princess Eugenie and Jack Brooksbank.
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Like her sister, Pippa hasn't disclosed the sex of the baby.
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She said: "James and Pippa have had a baby boy".
Pippa married James in a private ceremony in May past year in Englefield, south west of London.
"Kate and Pippa have had so much fun shopping online for baby clothes and accessories!" the insider added.
Swimming has been a major outlet for fitness fanatic Pippa throughout her pregnancy, during which she had to tone down her "five-day-a-week [exercise] routine".