Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 1101hrs.— Kensington Palace (@KensingtonRoyal) April 23, 2018
The baby weighs 8lbs 7oz.
The Duke of Cambridge was present for the birth.
Her Royal Highness and her child are both doing well.
Earlier Monday, Britain's royal palace confirmed the baby's arrival on Twitter. The baby is the third child for Kate and husband Prince William and fifth in line to the throne.
The baby, whose name has yet to be announced, weighed 8 pounds, 7 ounces and was born at 11:01 a.m. (6:01 a.m. ET|3:01 a.m. PT).
The Queen, The Duke of Edinburgh, The Prince of Wales, The Duchess of Cornwall, Prince Harry and members of both families have been informed and are delighted with the news.— Kensington Palace (@KensingtonRoyal) April 23, 2018
The 36-year-old duchess and her husband had traveled by car from their Kensington Palace home to the private Lindo Wing of St. Mary's Hospital in central London.
William and Kate married in 2011 and have two other children: Prince George, 4, and Princess Charlotte, who turns 3 next month. Both were born at the same hospital, as were William and his younger brother Prince Harry.
RAW VIDEO: Prince George and Princess Charlotte arrive at hospital to meet baby brother
The baby is Queen Elizabeth II's sixth great-grandchild, in line to the throne after grandfather Prince Charles, father Prince William and the two older siblings. The new arrival bumps Prince Harry to sixth place in the line of succession.
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The baby arrives just a few weeks before the wedding of uncle Harry to Meghan Markle on May 19.
In a mix of royal tradition and modernity, the birth was announced with a notice placed on an easel in the forecourt of Buckingham Palace - and on royal social media accounts.
Many are betting on a traditional royal name for the baby. Monday is St. George's Day, England's national day, but the name is already taken by the baby's big brother.
Like its older siblings, the baby's full title will be Her (or His) Royal Highness, Princess (or Prince), followed by several given names.
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The Associated Press contributed to this report.