The British royal shared a sweet photo of the couple, who were married in October, on Instagram, showing them beaming as they appeared to pose for a selfie.
The princess was complementary in the caption, extending happy birthday wishes to “my one and only Jack.”
“You are one of a kind and totally extraordinary. Nine birthdays and counting ... to quote you from a few months ago ‘you light up my life,’” she wrote, adding a few birthday-themed emojis.
The pair tied the knot in a ceremony at St. George's Chapel on the grounds of Windsor Castle.
Eugenie is the granddaughter of Queen Elizabeth II. She and her sister Beatrice are the daughters of Prince Andrew and Sarah Ferguson, who are divorced.
Fox News’ Sasha Savitsky and Kathleen Joyce contributed to this report.