Taken at his christening last week, the photograph follows a set of family portraits released by the royal family earlier.
The christening of Prince Louis has been marked by the release of a collection of family photographs.
The official portraits of the family are incredibly cute but the image that has captured our hearts is the sneaky extra the palace says we were never meant to see. Seated, left to right: Prince George, Prince William, the Duke of Cambridge; Prince Louis; Kate, the Duchess of Cambridge; and Princess Charlotte.
In the most intimate picture, the Duchess of Cambridge poses with Prince Louis in the gardens of Clarence House.
The picture, which was meant to be part of a private collection for Kate and Prince William - shows a giggling 11-week-old Louis in his mother's arms as they both smile at someone standing out of shot. Prince William and Kate have invited guests from the christening to Prince Charles and Camilla, Duchess of Cornwall's official residence for tea and cake.
However, in 2008, Queen Elizabeth ended that tradition when she commissioned her dressmaker to make a replica in order to preserve the original gown.
He is a younger sibling to Prince George, 4, and Princess Charlotte, 3 and is fifth in line to the throne.
She wore the ring on her right hand for the Duke and Duchess of Sussex's ceremony, and paired it with an ivory Alexander McQueen coat and dress.
Prince William and Kate welcomed their first child on 22 July 2013.
The photographs were released by the Duke and Duchess of Cambridge to mark the milestone.