The birth announcement, via a press release from Kensington Palace, said the boy was born at 4:24 p.m. weighing 8 pounds 6 ounces.
"Her Royal Highness and her child are both doing well and will remain in hospital overnight," it said. "Members of both families have been informed and are delighted with the news."
According to People, Queen Elizabeth II, Prime Minister David Cameron and Middleton's parents were among the first to be told the news.
The announcement was later confirmed by the Clarence House on Twitter.
Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 4.24pm— Clarence House (@ClarenceHouse) July 22, 2013
Buckingham Palace made the announcement in the form of a wooden easel placed outside the palace walls.
It's a time-honored royal tradition -- the birth of William's father, Prince Charles, was announced the same way in 1948. This time, however, officials have adapted to modern times by adding a simultaneous announcement on Twitter, so that millions around the world can get the news within seconds.
The Associated Press contributed to this story.