The photo shows the bride sitting on the floor as Princess Charlotte, 3, beams.
Savannah Phillips, 7, can be seen with her arm around Eugenie as the other flower girls watch on.
The candid shot was taken by wedding photographer Alex Bramall, in between the official portraits taken following the ceremony at St George's Chapel.
Eugenie's wedding was a huge success, with parents Prince Andrew and Sarah Ferguson putting on a very united front for the big day.