The boy, whose name has not been announced, weighed 8 pounds, 7 ounces, according to Kensington Palace.
William, 35, was seen leaving St. Mary’s Hospital just a few hours after the baby’s birth was announced publicly.
William’s return to the hospital with his two older children was greeted by cheers from the crowds of fans gathered outside St. Mary’s awaiting the new baby’s birth.
The new baby is now fifth in line to the British throne, following George, third in line to the throne, and Charlotte, fourth in line to the throne.
George and Charlotte both currently attend school near Kensington Palace, George at Thomas's Battersea and Charlotte at the Willcocks Nursery School. It is expected that Kate and William will initially enjoy solo time with their new infant at the palace.
Apartment 1A in Kensington Palace in London is the family's official home.