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.