Royal Baby Name Revealed
Kensington Palace announced in a tweet: "The Duke and Duchess of Cambridge are delighted to announce that they have named their son Louis Arthur Charles.
"The baby will be known as His Royal Highness Prince Louis of Cambridge."
The new prince, who was born weighing 8lb 7oz at 11.01am on Monday, is the couple's third child and fifth-in-line to the throne.
He was shown to the world by proud parents Prince William and Kate on the steps of the Lindo Wing at St Mary's Hospital in Paddington, west London, just hours after his birth on St George's Day.
William joked he now had "thrice the worry" as he held up three fingers to gathered reporters, but described both himself and his wife as "very happy".
Prince George, four, and Princess Charlotte, two, had earlier visited their new baby brother.
Kensington Palace has said there will be no extra childcare for the new baby, with William to spend a few weeks at home helping Kate.
The new prince's great-grandparents, the Queen and Prince Philip, his grandparents Prince Charles and the Duchess of Cornwall, and uncle Prince Harry were described as "delighted" with the baby's arrival.
The new prince's maternal grandparents, Carole and Michael Middleton, were not spotted visiting the Lindo Wing but were expected to meet their new grandson at Kensington Palace.
Carole was seen stocking up on supplies at a Boots store in west London within hours of the birth of the new baby on Monday.