The beautiful image was snapped in 2019 when she went to the Royal Ascot.
Eugenie looked stunning on the day wearing a gorgeous deep green dress with a peacock feather hat.
Eugenie's admirers came out in force, with hundreds taking to the comments section of the Instagram post to write their own Happy Birthday messages.
Eugenie's mother Sarah Ferguson also took to her personal Instagram to share a message for her daughter's big day.
"The joy of children," she wrote.
"In deep gratitude for luck in my glorious girls. Happy birthday Eugenie."
The post was accompanied by two gorgeous snaps of Sarah and her daughter when she was younger.
While Eugenie remains on maternity leave, it's likely we won't see much of her over the coming months.
But with the Princess being so active on her own Instagram account, we've no doubt she'll share some more anecdotes from her new life as a mum in no time.
This article originally appeared on our sister site, Now To Love.