The Duchess of Cambridge has given birth to a son weighing 8lbs 7oz, Kensington Palace has announced.
Kate was taken to the private Lindo wing of St Mary's Hospital in Paddington, London, earlier today.
The baby prince will be fifth in line to the throne, the Queen and the Duke of Edinburgh's sixth great-grandchild and a younger brother or sister for Prince George and Princess Charlotte.
Kensington Palace said: "Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 1101hrs. The baby weighs 8lbs 7oz.
"The Duke of Cambridge was present for the birth. Her Royal Highness and her child are both doing well."
The duchess is being cared for by consultant obstetrician Guy Thorpe-Beeston, who is the surgeon-gynaecologist to the household, and consultant gynaecologist Alan Farthing, the Queen's surgeon-gynaecologist - part of the trusted team who delivered George and Charlotte.
For her previous births, Kate had a 23-strong team of top medics working or on stand-by from Imperial College Healthcare NHS Trust - which runs St Mary's Hospital.
Theatre staff were ready, along with a lab technician, replacement anaesthetists and paediatricians, a back-up for the consultant, and workers from a special baby care unit.
The press pen opened outside the hospital earlier, as journalists, photographers and camera crews waited for confirmation of the new baby.
With George, the duchess gave birth 10-and-a-half hours after being admitted to hospital, but Charlotte was born just two hours and 34 minutes after Kate arrived at the Lindo Wing.
The birth will be declared in both conventional and contemporary ways.
Predicted names Arthur, Albert, Frederick, James and Philip are popular for a boy.
George and Charlotte may visit the hospital to see their mother and baby sibling, just as George did when his younger sister was born three years ago.
But it depends how quickly Kate wants to head home.
The duchess's pregnancy was announced by Kensington Palace on September 4 last year.
Kate again suffered from extreme pregnancy sickness hyperemesis gravidarum.
She went on maternity leave on March 22, but the duchess - and her large bump - attended an Easter Sunday church service in Windsor with the royals on April 1.
She has joked that William is "in denial" about having a third child.
Although the Queen has four children, the duke is the first future king to have a third for more than 120 years.
In 1897, George V, then the Duke of York, welcomed a daughter - Princess Mary - the younger sister of Edward VIII and George VI.
Unlike William, George V never expected to be king and only became heir to throne when his older brother, the Duke of Clarence and Avondale, died from flu in 1892.
William and Kate follow in the footsteps of the Queen and the Duke of Edinburgh, who had a son, a daughter and then another son.