In the picture, Princess Beatrice can be seen looking shocked while Karl shows off her huge diamond ring.
While Beatrice, who's official title is Beatrice, Princess of York, is not a working royal she is allowed to have social media accounts. However, the princess has chosen to keep it private, with only 600 people following her.
While Princess Beatrice is not known to be dating anyone at the moment, the princesses ex-boyfriend of ten years Dace Clark recently wed partner Lynn Anderson in a lavish Italian wedding.
At the time of their split, many reports noted that Beatrice and Dave decided to part ways after the failure of long and exasperated 'mark or break talks' centred on whether or not they should marry.
A source close to the 29-year-old royal, told The Mail on Sunday at the time: 'After ten years together, Beatrice and Dave had to wonder why they were not married.
'They talked it through and decided to take time apart to reflect on the future. They remain friends and the decision to break up was mutual.'
Princess Beatrice was in St Tropez with her mother, Sarah Ferguson, at the time of the wedding.