Prince Louis Arthur Charles' birth was officially registered on Tuesday morning.
The Duke and Duchess of Cambridge's third child was born on April 23, and now the newborn's proud father has signed the birth register at Kensington Palace, witnessed by Westminster City Council registrar Patricia Gordon.
The official document reveals the little tot's full name as His Royal Highness Prince Louis Arthur Charles of Cambridge.
It also lists the occupations of William, 35, and wife Catherine, 36 - who also have Prince George, four, and two-year-old Princess Charlotte - as Prince and Princess of the United Kingdom.
Registrar Mrs. Gordon said: "It has been a great honour to register, and formally welcome the latest addition to the royal family, Prince Louis Arthur Charles.
"This follows a long history of Westminster City Council registering royal births, including Prince Louis' siblings Prince George and Princess Charlotte."
The royal couple's little boy, who weighed 8lbs 7oz at birth, takes one of his names from his grandfather, Prince Charles.
Revealing the baby prince's moniker after weeks of speculation, Kensington Palace confirmed: "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."
Announcing the child's birth, they wrote: "Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 1101hrs.