Middleton donned a bright red Jenny Packham gown to the event, the same one she wore to a China State Dinner last October, which she styled with stunning diamond earrings and a matching bracelet.
But all eyes were on her sparkly headpiece, the Cambridge Lover's Knot tiara, which was one of the late Princess Diana's favorites. The diamond and pearl-encrusted accessory was reportedly given to Diana by the queen as a wedding present in 1981.
The 34-year-old royal also posed for a new family portrait with her husband, Prince William, Camilla, Duchess of Cornwall, Prince Charles, Prince Philip and Queen Elizabeth II, all who looked equally regal. Like Middleton, the Queen and Duchess of Cornwall were rocking sparkly tiaras of their own!
The same gorgeous photo was later posted to the Kensington Palace Instagram, accompanied by a caption which read, "The Duke and Duchess of Cambridge join The Queen and the Royal Family as they welcome the world's Ambassadors and High Commissioners from the UK's diplomatic community to Buckingham Palace for the annual Diplomatic Reception."
This marked Middleton's fifth time wearing a tiara since becoming part of the royal family in 2011. Middleton also sported a tiara on her wedding day (opting for the Cartier Halo headpiece), as well as to the aforementioned China State Dinner, last year's diplomatic dinner and a state reception in 2013.
Watch the video below to revisit Middleton's full look from last year's Diplomatic Reception.