The New York City-based hedge fund analyst popped the question to during a romantic holiday to the island of Ithaca in Greece.
Nina, who serves as the creative director for her father's airline Vistajet, took to Instagram to share the happy news with a pictureseque photo of the couple at sunset.
Though we haven't gotten a close-up of the ring, it appears to feature a deeper gemstone, surrounded by a diamond halo on a silver or platinum band.
Philippos is the youngest son of King Constantine, godfather of Prince William, and Queen Anne-Marie, whose sister is Queen Margrethe II of Denmark.
"Their Majesties King Constantine and Queen Anne-Marie are delighted to announce the engagement of their youngest son HRH Prince Philippos to Nina Nastassja Flohr, daughter of Thomas Flohr and Katharina Flohr," the family's office shared in an official statement.
The family have retained their royal titles even after the abolition of the Greek monarchy in 1973.
Their ties to the British Royal Family date back far, with the families even going on vacation together when Prince William and Prince Harry were kids.
As to where, or even when, the couple will wed remains to be confirmed. However, one look at their Instagram pages tells us that they love a good getaway, so we're hoping for a dreamy destination wedding.
With Nina being quite the fashionable royal-to-be, we anticipate one stylish wedding, full of famous guests.
Philippos' wedding isn't the only one to look forward to, though. His older sister, Princess Theodora, is also currently engaged to Los Angeles-based attorney Matthew Kumar. The pair have postponed their wedding to the pandemic.