It is a boy for the Duke and Duchess of Cambridge.
Kate, the wife of Britain's Prince William, gave birth Monday to their third child, a boy weighing 3.8 kilograms.
Well-wishers had camped for several days outside St. Mary's Hospital in London to await the birth of the newest British royal.
There has been no word on the baby's name.
"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 said in a statement on Twitter.
British Prime Minister Theresa May welcomed word of the baby's birth.
About seven hours after the birth, the couple emerged from the hospital, with Kate holding the infant in a white blanket.
The couple stood on the steps of the hospital wing, waving to well-wishers, before heading back inside.A short time after that, Kate and William left the hospital with the baby.
William and Kate have been married since April 29, 2011. Their two other children: Prince George, 4, and Princess Charlotte, who turns 3 next week, were born at the same hospital.
The new addition to the British royal family is fifth in line to the throne.
In the past, the rules of succession favored male heirs, but that policy changed in recent years so royal daughters would have the same rights as sons.William's younger brother, Harry, is now sixth in line.
The new baby's arrival comes less than a month before Harry marries U.S. actress Meghan Markle.Their wedding is set for May 19 at St. George's Chapel, Windsor Castle.