The queen has made the duchess a Dame Grand Cross of the Royal Victorian Order for her "services to the sovereign," Buckingham Palace announced.
The award, which was personally selected by the queen, is the equivalent to being made a knight. The Royal Victorian Order was established in 1896 by Queen Victoria.
Kate reportedly will be officially honored at this year's service for the order on May 3 at Windsor Castle.
Her new honor also comes with a visible distinction, as those who receive it wear a blue sash at formal events in acknowledgement of their appointment. Kate will wear the sash in addition to the yellow badge signifying the Royal Family Order that she wore in October at an event in Buckingham Palace.
The queen's decision to bestow the honor appears to be her special way of marking Kate's wedding anniversary. Camilla, Duchess of Cornwall, also received the same honor in 2012 on her seventh wedding anniversary.
The duchess, 37, and the queen, 92, have often been seen in public enjoying each other's company. Last month they made a rare joint appearance when they visited a new facility at King's College London.
The honor bestowed by the queen comes as Kate and Prince William celebrate a special day, as it's been eight years since their fairy-tale wedding.
Kate hearkened back to that magical day earlier this month when she wore the earrings from her wedding to Easter services at St. George's Chapel.
The couple now have three children, Prince George, 5, Princess Charlotte, 3, and Prince Louis, who celebrated his first birthday last week. In a tribute to the queen, Princess Charlotte's middle name is Elizabeth.
The duchess will also soon become an aunt again. Prince Harry's wife, Meghan, Duchess of Sussex, is set to give birth any day now.