The card reads, "Thank you from the bottom of our hearts for making our wedding so special and for thinking of us as we start our married life,' the couple wrote. 'We are completely overwhelmed with happiness. Eugenie and Jack".
Speaking to FEMAIL, etiquette expert William Hanson said: "Gratitude is dying in modern Britain, but I am so glad to see that it is alive and well in the Royal Household.
"The letters from Princess Eugenie and Mr Brooksbank are charming examples of the perfect thank you letter. They are not too short, but not too long; succinct and heartfelt.
"They show that the couple has excellent manners."
On November 5, Eugenie was spotted out for the first time since her big day.
The royal stepped out to attend the Casamigos Day of the Dead party at Isabel's in Mayfair.
Sporting a huge smile, the 28-year-old opted for an all black look and a bold red lip.
Eugenie paired her outfit with navy velvet booties and her huge engagement ring.