Prince William said he and his wife, the former Catherine Middleton, "could not be happier" about the birth of their baby boy.
Queen Elizabeth II and Prince Philip, the baby's great-grandparents, were "delighted."
Prince Charles and his wife, Camilla, the Duchess of Cornwall, were "overjoyed."
But what about the rest of the world?
From across the British Commonwealth and beyond, messages of congratulations poured in Monday, even as the speculation quickly moved on to when the baby would make his first public appearance and what name he would be given.
Twitter lit up with news of the highly anticipated royal birth. And in a decidely modern twist, Clarence House responded with a tweet inviting people to sign a Google+ card for the couple.
Want to send your congratulations to Their Royal Highnesses? You can sign the Google+ card here: http://t.co/mMgjmPzf3s— Clarence House (@ClarenceHouse) July 22, 2013
"Michelle and I are so pleased to congratulate the Duke and Duchess of Cambridge on the joyous occasion of the birth of their first child," Obama said in a statement. "We wish them all the happiness and blessings parenthood brings.... Given the special relationship between us, the American people are pleased to join with the people of the United Kingdom as they celebrate the birth of the young prince."
Earlier, as royal watchers around the world waited for word of the birth, the White House tweeted:
British Prime Minister David Cameron called the royal birth "a wonderful moment for a warm and loving couple who have got a brand new baby boy."
"It has been a remarkable few years for our royal family -- a royal wedding that captured people's hearts, that extraordinary and magnificent jubilee and now this royal birth -- all from a family that has given this nation so much incredible service," Cameron told reporters in London. "They can know that a proud nation is celebrating with a very proud and happy couple tonight."
The archbishop of Canterbury, Justin Welby, said: