The prince, who was born at 11.01am on Monday 23 April weighing 8lbs 7oz, will be known as His Royal Highness Prince Louis of Cambridge.
The name Louis is of German and French origin meaning "renowned warrior". It is currently 71st most popular boys' name in the UK, according to the Office for National Statistics and is now likely to see a surge in popularity. Louis is also one of Prince George's middle names.
The announcement shocked royal fans, who assumed the most likely name for the third royal baby would be Albert, Arthur or James.
Louis' older brother's full name is George Alexander Louis, who has the same first name as his great-great-grandfather (the Queen's father) King George VI. It is also one of his grandfather Prince Charles' middle names and of course, it is also the name of the patron saint of England, who legend holds it overcame a fire-eating dragon.
Middle child, Princess Charlotte's full name is Charlotte Elizabeth Diana, which could also be a tribute to Prince William's father Prince Charles as Charlotte is the feminine form of the name Charles. Her middle names are those of her great grandmother Queen Elizabeth II and her grandmother Diana, Princess of Wales.