The Duchess of Cambridge safely delivered the baby, weighing 8 lbs 3 oz., early Saturday at St. Mary's Hospital.
PHOTOS: Duchess of Cambridge gives birth to baby princess!
Britain's newest royal is fourth in line to the throne and the fifth great-grandchild of Queen Elizabeth II, 89.
Crowds of well-wishers outside the hospital cheered as the couple stepped out briefly, posing and waving for photographers.
VIDEO: Prince William, Kate Middleton introduce newborn princess to the world
Kate, who wed William in April 2011, gave birth to their first child, Prince George, at the same hospital in July 2013.
PHOTOS: Kate, Duchess of Cambridge, throughout her pregnancy
Her Royal Highness The Duchess of Cambridge was safely delivered of a daughter at 8.34am.— Kensington Palace (@KensingtonRoyal) May 2, 2015
The baby weighs 8lbs 3oz. The Duke of Cambridge was present for the birth.— Kensington Palace (@KensingtonRoyal) May 2, 2015
Her Royal Highness and her child are both doing well.— Kensington Palace (@KensingtonRoyal) May 2, 2015
Here is is. The official proclamation. William and Kate give birth to Princess. Posted at Buckingham Palace pic.twitter.com/bdhqMN2LfB— Carolyn Durand (@CarolynDurand) May 2, 2015