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.
Clarence House, which is the official home of Prince Charles and his sons, also invited parents of other new arrivals to share their photos using #WelcometotheWorld.From America, President Obama said the child “enters the world at a time of promise and opportunity for our two nations.”
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:
Oh, hello there! pic.twitter.com/WvoZ2WiIrs— The White House (@whitehouse) July 22, 2013
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:
"I am delighted to congratulate the Duke and Duchess of Cambridge on the arrival of their baby boy. Along with millions here and around the world, I share in their joy at this special time. May God bless this family with love, health and happiness in their shared life ahead."
Prime Minister Stephen Harper of Canada tweeted:
"This is a happy day for our close friends in Britain and the Commonwealth," Australian Prime Minister Kevin Rudd said in a statement.
"Prince William holds a special place in the hearts of many Australians. Some of us remember his first visit to this country as a 9-month-old baby more than 30 years ago with his mother Diana and father Charles. In more recent times, Prince William demonstrated deep compassion when he met with families in Victoria who had lost everything during the Black Saturday bushfires of February 2009. We share in the joy of the Royal Family, particularly Prince Charles on the birth of his grandson and Queen Elizabeth II on the birth of her great-grandchild."
But for those getting a little tired of the international media frenzy about the royal baby, the Guardian newspaper in London provided some relief. By clicking on "Not a royalist?" readers could opt out of seeing the minute-by-minute coverage that dominated British and American media sites.