Her Royal Highness is sure to be delighted with the moniker, but the revelation has raised questions about Eugenie's relatives in the British royal family - specifically regarding their surnames (or lack thereof).
The answer dates back over 100 years to 1917!
Before then, members of the royal family didn't have last names, with monarchs simply going by their first name and the country they ruled, KXNet.com reports.
On July 17, 1917, Queen Elizabeth II's grandfather King George V announced all direct descendants of Queen Victoria would use the name Windsor as their official surname from then on.
But in 1960, 13 years after their royal wedding, the Queen and the Duke of Edinburgh decided to carve their own piece of history by changing the family's last name to Mountbatten-Windsor.
Philip, who was Prince of Greece and Denmark before his marriage to Elizabeth, used the name Mountbatten in honour of his maternal grandparents who changed their last name from Battenberg to Mountbatten during the First World War, according to the Royal Family's website.
The Queen's descendants are all entitled to use the hyphenated name, as Princess Anne did on her marriage certificate in 1973.
But as The Independent notes, the young royals of today "don't really need a last name" as their official titles are already famous enough and long enough - think His Royal Highness Prince William, Duke of Cambridge and His Royal Highness Prince Harry, Duke of Sussex
However the confusion doesn't stop there, as the Royal Family's website explains those part of the Queen's family tree may use their family's "territorial designation" instead.
Prince Charles could use Wales, Princesses Beatrice and Eugenie could opt for York, and Kate Middleton's children Prince George, Princess Charlotte and Prince Louis could all refer to themselves as Cambridge.
Charles and Princess Diana's sons used their geographical names during their time in the military, where they went by William and Harry Wales.
Other members of the royal family have taken matters into their own hands, like Princess Anne's children Zara and Peter who hold no royal title and go simply by their father's last name - Phillips.
Another variation is Prince Edward and Sophie, Countess of Wessex's daughter Lady Louise who only uses her great-grandmother's surname, Windsor.
Still, it seems Queen Elizabeth and Prince Philip's efforts to unite the House of Windsor and Mountbatten were successful, a few generations down the line at least.
In May, Prince Harry and Meghan Markle announced the name of their firstborn son: Archie Harrison Mountbatten-Windsor.
The Royal Family's website says: "Unless the Prince of Wales chooses to alter decisions when he becomes King, he will continue to be of the House of Windsor and his grandchildren will use the surname Mountbatten-Windsor."
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