The Duchess of Cambridge has given birth to a girl, Kensington Palace has announced.
The princess - who is fourth in line to the throne - was "safely delivered" at 08:34 BST, the palace statement said.
The Duke of Cambridge, who was present for the birth of the 8lbs 3oz (3.7kg) baby girl, brought his 21-month-old son Prince George to visit his sister at St Mary's Hospital, west London.
The couple and their daughter will leave hospital on Saturday evening.
They will travel home to Kensington Palace, where Prince George has now returned.
In a statement, Kensington Palace said: "Their Royal Highnesses would like to thank all staff at the hospital for the care and treatment they have all received.
"They would also like to thank everyone for their warm wishes."
As he left the hospital briefly to pick up his son, Prince William told the waiting crowds the couple were "very happy".
Both Catherine and her daughter are "doing well", Kensington Palace said. The name of the baby will be announced in due course.