Her other photo, where she is also grinning, shows her heading into St. George's Chapel at Windsor Castle for her walk down the aisle.
Incredibly, the photos are almost identical and fans can't cope.
"Awww! I guess you picked out your dress and flowers a little while ago," one fan said.
"Omg the similarities of the bouquets," another said.
"I see no difference," someone else mused.
"Same smile …"
"You look so pretty and beautiful in both photos."
There were a few key changes though, such as her flower headband, which was replaced with the Greville Emerald Kokoshnik tiara borrowed from Queen Elizabeth on her actual big day.
She also ditched the veil on her wedding day, something she decided to forgo to show off a scar on her back stemming from a childhood scoliosis surgery.
The royal chose a bridal gown with a low-cut back for the same reason.
Her adorable post has already clocked more than 95,000 likes in just hours.
"I had always wanted a low back — part of it was showing my scar, and I believe scars tell a story about your past and your future, and it's a way of getting rid of a taboo," she told People.
"For me, it's a way of communicating with people who are going through either similar situations with scoliosis or having a scar of their own that they are trying to deal with."
She added she received a lot of letters from people who were happy she had stood up and showed her scar.
Eugenie's gown featured a fitted bodice and full pleated skirt and was designed by Peter Pilotto and Christopher De Vos, who founded the British-based label Peter Pilotto in London in 2007.
