She joined husband William and his brother Prince Harry for a special lunch at Windsor Castle during which Queen Elizabeth entertained monarchs from 26 countries and most members of her immediate family.
Kate, 30, wore a $2,000 coat dress designed by Emilia Wickstead. Described as a pleated dress with a square neck, it is made of double crepe wool and is from the current collection.
The stylish Kate was seen chatting with William, 29, and Harry, 27, in the Waterloo Chamber at the Castle before a lunch of English asparagus tartlet, new season Windsor Lamb with braised potatoes and vegetables – and a tomato and basil salad.
Dessert was strawberries and vanilla charlotte.
Guests included Prince Albert and his wife Charlene; the former king of Greece; the kings and queens of Denmark, Norway and Sweden; and King Abdullah and Queen Rania of Jordan.
Among the younger British royals: Princesses Beatrice and Eugenie, who attended with their father Prince Andrew. Later, Prince Charles and his wife Camilla are hosting a dinner for the visiting sovereigns back in London at Buckingham Palace.
For more on Kate’s sexy style, pick up the latest issue of PEOPLE, on newsstands now