Kate, 37, donned the £1,750 Alessandra Rich frock for her outing to a D-Day exhibition at Bletchley Park, the home of the Second World War codebreakers.
Princess Diana wore her take on the feminine look for photos taken in her living room at Kensington Palace in October 1985.
Social media users were quick to note Princess Diana owned a similar midi dress, which she wore for a photo call at Kensington Palace in 1985.
Several praised Kate for the subtle tribute to her late mother-in-law, while one said it proved 'good taste never goes out of style'.
Another wrote: 'I think it is a nice way to pay tribute to her mother in law she never got to meet.'
A third added: 'I absolutely LOVE all the "subtle" tributes to Princess Diana. She was beautiful, kind and loved by many.'