Eight-week-old Louis, a younger brother to Prince George and Princess Charlotte, is fifth in line to the throne and a great-grandchild of the Queen and the Duke of Edinburgh.
The Royal Family Instagram account shared a photo of William with his grandmother the Queen, 92, with a similar celebratory message.
Amid the chaos it's easy to forget, but yes, there was a new royal baby born in April (and one yesterday, but that's beside the point), and Kensington Palace announced that he'll officially be baptized on July 9.
What will Prince Louis wear at his christening?
"Judging by the previous christenings, and the importance of the day for the family, Louis' christening will be a private affair, perhaps no surprise there".
George's christening at the same venue in October 2013 was an intimate occasion, with only 23 guests attending.
Charlotte's baptism in 2015 involved the Cambridge family walking to church past well-wishers.
So far, the public have had just two glimpses of Kate and William's youngest child.
Prince Louis was last seen shortly after his birth in April when he was taken home by his parents, Kate Middleton and Prince William, the Duke and Duchess of Cambridge. Welby in May performed the wedding of Harry and Meghan, Duke and Duchess of Sussex.
The lovely picture, which was taken by the Duchess, showed the baby Prince dressed in delicate white and propped up on a cosy cushion.