The youngest child of the Duke and Duchess of Cambridge wore a sailor-style white and navy top, very similar to the one worn by William at Trooping the Colour in 1985.
Louis, dressed in his potentially vintage outfit, sat in between his older siblings Prince George and Prince Charlotte as they travelled to Horse Guards Parade for the first event of the four-day celebration.
Pictures of William wearing the same style of top show the young prince on the balcony of Buckingham Palace standing in front of his parents in 1985.
The Prince of Wales is holding a young Harry while Diana, Princess of Wales, is smiling.
William was almost three at the time.
On Thursday, four-year-old Louis joined his brother and sister in waving at the crowds from their carriage.