Prince William's wife, Kate, delivered a baby boy this afternoon in a private wing of a central London hospital, palace officials said.
He came in at 8 pounds, 6 ounces.
The official announcement: "Her Royal Highness and her child are both doing well and will remain in hospital overnight."
The boy's name hasn't been released yet.
It was a historic moment for the British monarchy — the couple's first child is third in line for the British throne, after Prince Charles and William, and should eventually become a monarch.
William and Kate, both 31, entered St. Mary's Hospital in central London through a side door early Monday, avoiding the world's media. Palace officials confirmed her arrival about 90 minutes later.
Royal officials said they traveled by car, without a police escort, just before 6 a.m.
Kate — also known as the Duchess of Cambridge — gave birth in the private Lindo Wing of the hospital, where Princess Diana also gave birth to William and his younger brother, Prince Harry.
Kate was looked after by a top-notch medical team led by royal gynecologist Dr. Marcus Setchell. Palace officials did not provide many details about how her labor progressed.
The baby is in line to become Britain's monarch and be recognized as head of state in 15 other countries, including Canada and Australia, playing a leading role in charting the future of the Commonwealth.
Royal watchers had waited to be told of the baby's arrival from the palace, which revealed the news through a mixture of tradition and social media.
Palace officials said that the first hint came when a royal aide emerged from the hospital with a signed bulletin carrying the Buckingham Palace letterhead. The bulletin was given to an official who was driven to Buckingham Palace, where it was posted on an easel in public view in front of the building.
At the same time the bulletin was posted, there was an official announcement on Twitter and the media was formally notified.
It is the baby's gender that is of particular interest because the prospect of Kate's pregnancy prompted a change to laws of succession to ensure a daughter would not be passed over for the crown by a younger brother.