The child, who was born in the Lindo Wing of St Mary's Hospital, central London, is fifth in line to the throne and the Queen's sixth great-grandchild.
The new arrival, who is the third child of Catherine and the Duke of Cambridge, was born at 11:01 BST weighing 8lbs 7oz.
A Kensington Palace statement said Catherine and her new son are "both doing well".
William was present for the birth, the palace added.
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