While at an Anzac Day service in London on April 25 before the moniker was shared publicly, William chatted with other attendees and was heard hinting to one that his new son had a "strong name", the Daily Mail reported.
We'll have to wait and see if the name Louis sees a spike in popularity now that it has the royal seal of approval.
William and Kate's choice has come as a surprise as it is already one of Prince George's middle names, and also the duke's.
The brief moment led royal watchers to speculate that Arthur, which is also a royal name, could be a part of the newborn's name. According to CNN, Lord Mountbatten claimed to be a "mentor" to Prince Charles, before he was killed by the IRA in Ireland in 1979.
"The Duke and Duchess of Cambridge are delighted to announce that they have named their son Louis Arthur Charles", said a palace statement. Prince Charles, the baby's grandfather, is first in line to the throne.
Prince Charles arrived back in the United Kingdom on Thursday, but is still yet to travel down to London to meet the new addition to his family. Charlotte Elizabeth Diana, 2, was named after her grandfather Prince Charles, her great-grandmother Elizabeth and her late grandmother Diana.
Only by the fact that they both have the same name - Louis!
Signed by royal mom and dad Prince William and Kate Middleton, whose full titles are printed on the certificate, their out-of-ordinary jobs are also revealed: Prince of the United Kingdom and Princess of the United Kingdom.
The future King and his wife, Camilla, Duchess of Cornwall, were at their private Scottish home, Birkhall on Monday, when the Duchess gave birth.
Meanwhile, Prince William thanked the medical staff who helped deliver his son. Louis was an unlikely bet, the name taking tenth place with bookies but the underdog won. Duchess of Cambridge, who was pregnant with first child Prince George at the time, and the Duke of Cambridge even got into a wand duel, the couple checked out Diagon Alley along with Prince Harry.