The third in line to the throne, who was born on Monday at 16:24 BST, will be known as His Royal Highness Prince George of Cambridge.
George was the bookmakers’ favourite for the first name of the prince.
The duke and duchess had been expected to pick a traditional royal name for their son, who was born at St Mary’s Hospital in London, weighing 8lbs 6oz.
Royal infants usually have historical names which are passed down through the generations.
There have been six King Georges up to now, most recently the Queen’s father, although his first name was Albert and he was known to his family as Bertie.
The name Louis is Prince William’s fourth name and is likely to be a tribute to Lord Louis Mountbatten, the Duke of Edinburgh’s uncle and the last British Viceroy of India before independence in 1947.