In the photo, the fresh-faced couple are all smiles as they pose for the candid snap in their race-day finery.
Enthusiastic fans were quick to take to Instagram to comment on the snap, with one person writing: “Lovely picture of a stunning couple!”
Another person stated: “That's lovely, you were so young! Wishing a very happy Valentine's day to you and Jack, and many, many more to come!”
A third person added: “Happy Valentine's Day to you both.”
Eugenie’s sweet Valentine’s Day gesture came after she took to Instagram just two days earlier to share another throwback snap of the pair on their engagement day.
“On this day two years ago... Jack and I announced we were engaged,” Eugenie captioned the snap, which showed them posing in the Picture Gallery at Buckingham Palace.
At the time, Eugenie stunned in a floral print Erdem dress, which she teamed with a pair of Jimmy Choo shoes.
Cosying up to her then fiancé, the princess flaunted her dazzling Padparadscha sapphire and diamond ring.
Euegenie and Jack later tied the knot at St George's Chapel in Windsor Castle on October 12, 2018.