The birth of the new heir was announced by Kensington Palace.
"Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 4.24 p.m.," the palace said in a statement. "The baby weighs 8 lbs. 6 oz. The Duke of Cambridge was present for the birth.
"The queen, The Duke of Edinburgh, The Prince of Wales, The Duchess of Cornwall, Prince Harry and members of both families have been informed and are delighted with the news," the statement said. "Her Royal Highness and her child are both doing well and will remain in hospital overnight."
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"We could not be happier," Prince William said in a short statement released by the palace. He will be spending the night in the hospital with his wife and new son.
The annoncement came after Middleton had been in the hospital for more than 14 hours, though the baby was born several hours before the birth was announced.
Cheers erupted at St. Mary's Hospital, where the baby was born, and Buckingham Palace when the announcement was released.
Throngs of people gathered outside of the palace to wait for the formal, signed birth announcement that was driven from the hospital to the palace. Following tradition, the document was taken into the palace to be shown to the queen before it was placed on an easel in the palace court.
The newborn is third in line to the British throne after his grandfather, Prince Charles, and father, Prince William. The reigning monarch, Queen Elizabeth II, has been on the throne 61 years. The baby is the queen's third great-grandchild.
"The Queen and Prince Philip are delighted at the news of the birth of The Duke and Duchess of Cambridge's baby," the British monarchy's official Twitter account posted.
Prince Charles expressed his joy about being a first-time grandfather.
"Both my wife and I are overjoyed at the arrival of my first grandchild," Prince Charles said in a statement. "It is an incredibly special moment for William and Catherine and we are so thrilled for them on the birth of their baby boy."
"Grandparenthood is a unique moment in anyone's life, as countless kind people have told me in recent months, so I am enormously proud and happy to be a grandfather for the first time and we are eagerly looking forward to seeing the baby in the near future," Prince Charles said.
The world's intense interest in the Duke and Duchess of Cambridge's real-life fairy tale extended to Middleton's pregnancy and the baby long before it was born.
"You have this gorgeous young couple," ABC News royal contributor Victoria Murphy told ABCNews.com. "He's a prince and she's a commoner, and they get married and this is their first baby -- so it's like the completion of the fairy tale, and I think people find that really fascinating.
"There's a lot of fascination internationally in the British royal family just because it's such an established institution and people find it quite quaint," Murphy said. "People are just generally really curious about it because it's not something that exists in every country."
Every moment leading up to the delivery has been closely scrutinized, including the royal bump, her pregnancy fashion and every baby-store shopping trip.
"It's the fascination and joy everybody feels when a baby is born and then, for British people certainly, the renewal of the monarchy," Robert Lacey, author of the book "Majesty," told ABCNews.com.
"We know that this new baby ... is our sovereign who will be our figurehead by the end of this century, maybe even into the next century," he said.
The British Parliament is changing a 300-year-old law so that William and Kate's firstborn, regardless of gender, would be the heir to the throne.
Queen Elizabeth II was only person eligible to be monarch because her father had no male children.
This birth marks the first time in almost 120 years that a reigning monarch, Queen Elizabeth II, 87, is alive at the same time as three generations of heirs: Prince Charles, 64, Prince William, 31, and, now, the latest addition to the family.
The baby boy will someday be the monarch of 16 sovereign states and commander of the British Army, among having other royal responsibilities.
"This new baby physically represents the mixture of history and new blood coming in, and the promise," Lacey said.
The roller-coaster-like relationship between the public and the royal family is at a high point right now, thanks, in great part, to Middleton and her grounded nature, Lacey said.
"You have to strike a balance between the dignities and traditions of the family and the monarchy with something new, and she's got that something new," Lacey said. "I think it's freshness, it's energy, it's directness, it's honesty. It's coming from a hardworking family which, to use an English expression, pulled itself up by its bootstraps."
The royal couple announced the pregnancy Dec. 3, 2012, when the Duchess, 31, was admitted to King Edward VII Hospital in central London with hyperemesis gravidarum, an acute morning sickness that requires supplementary hydration and nutrients.
Royal babies have typically been born within a year of marriage. Princess Diana gave birth to William 11 months after her wedding and the queen gave birth to Prince Charles six days before her first wedding anniversary.
Prince William and Kate were married April 29, 2011.
"I think this is something they've had in their minds that they really wanted to do and they're so excited that it's happening," Murphy said. "They've got friends who have kids, they're at a really great age, they've been together for a long time, they're still young enough to have lots of energy. I think both of them are just really, really excited about becoming parents."
It is widely expected that the new parents will strive for normalcy in their child's life, much like the prince's late mother, Princess Diana, instilled in her two sons, William and Harry. Diana famously made it a point of taking her boys beyond the palace walls for outings to the movies, amusement parks and McDonald's.
She also involved them in her charity work and famously took her young sons on a visit to a homeless shelter in 1996, the year before she died in a car accident. William is still involved with the shelter.
"She played a huge part in my life and Harry's growing up in how we saw things and how we experienced things," Prince William told ABC News last year of his mother. "She very much wanted to get us to see the role in this of real life and I can't thank her enough for that because reality bites in a big way and it was one of the biggest lessons I learned, is just how lucky and privileged how many of us are."
Such an attitude will likely influence the way William and Kate raise their new baby and future ruler.
"The baby is going to be a different sort of leader of the country in that the whole essence of the royal family nowadays succeeds or fails on its ability to communicate with the rest of the population," Lacey said.
"We're not looking for inspired, divine leadership. We're looking for common sense and humanity," Lacey said
"I think that will be the priority for the parents, to give the child the most normal possible upbringing not only for its own good, but for the good of the job it's got to do in the future. It's got to have the common touch."