Kate, the wife of Prince William, gave birth to a baby son on Monday, the couple's third child who is fifth in line to the British throne.
The boy, weighing 3.8kg, was born at 11:01am (1001 GMT), Kensington Palace announced.
Mother and baby are both doing well, the palace said, and William, the Duke of Cambridge, was present for the birth.
The baby is the couple's third child, after Prince George and Princess Charlotte.
Queen Elizabeth II, William's father Prince Charles, and his brother Prince Harry were said to be delighted with the news.
"Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 11:01 hours," the palace announced in its brief statement.
Kate, 36, was admitted early Monday to the private Lindo Wing of St. Mary's Hospital in central London, where four-year-old George, and Charlotte, aged two, were born.
Royal fanatics, who had been camped outside the hospital wing for several days in anticipation, jumped for joy and popped open champagne.
"We're going to celebrate with fish and chips!" said John Loughrey, who was wearing a Union Jack hat and clutching a plastic doll with a crown.
Maria Scott, 46, a housewife from Newcastle in northeast England, has been in place for 15 days.
"It's really important to show support because they need to know how much they are loved by the people," she told AFP.
"We're very proud of our monarchy."
Tony Appleton, a town-crier dressed in a flamboyant red uniform, announced the news from the steps of the hospital's Lindo Wing to a crowd of supporters and media.
The baby was born on St. George's Day, England's national day.
The baby boy is a sixth great-grandchild for Queen Elizabeth, who turned 92 on Saturday, and her husband Prince Philip.
It will hold the title of prince after Queen Elizabeth changed the rules in 2012 to ensure that all of William's children would be entitled to the style, not just his eldest son. (AFP)