"Young Louis welcome to the world", he wrote, "I'll take you under my wing lad".
Bookmakers had been doing a brisk trade in bets on the new prince's name, but Louis wasn't among the favourites. He weighed in at 8 pounds, 7 ounces. William and Prince George have it as a middle name.
The couple stepped out of St Mary's Hospital in central London to cheers from a crowd of supporters and global media outlets gathered outside.
And then there's also a connection with the name to Princess Diana - as one of her nephews was named Louis Spencer.
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Louis joins siblings Charlotte, 2, and George, 4, in the growing royal family, and the queen, Philip, Charles, and his wife, Camilla have all expressed delight with the birth of the monarch's sixth great-grandchild.
Louis is somewhat of a family name for the Windsors.
"It has been a great honor to register and formally welcome the latest addition to the royal family, Prince Louis Arthur Charles", Gordon said in a statement. His second name, Arthur, is steeped in historical meaning and is also Prince William's middle name.
He's already had quite a few special visitors.
Charlotte turns three on Wednesday, her first birthday as an older sister and a middle child.
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There are others yet to visit little prince.
For their third child, William and Kate may have made a decision to wait for a few days to announce his name until they had a chance to introduce the baby prince to Prince Charles and Queen Elizabeth.
That's according to dad William, who said that Louis was "behaving himself" at an Anzac Day service. Two days after the birth of his son, he was seen nodding off during an Anzac Day service at Westminster Abbey.
Here are his famous fans.
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