The skeleton on display is of a whale that became stranded in Wexford Harbour in Ireland in 1891, 10 years after the opening of the Natural History Museum.
Kate and George had a memorable outing in 2015, where she was spotted holding his hand and showing him around the exhibits at the Natural History Museum.
The 82ft-long mammal, called Hope, is suspended from the Hintze Hall ceiling and was unveiled by the duchess and TV naturalist Sir David Attenborough. Turning heads as she passed by, the royal oozed elegance in a short-sleeved knee-length Preen by Thornton Bregazzi blue dress which retails at $683.
"It is within the grasp of humanity to shape a future that is sustainable, and now more than ever we want our galleries and exhibitions to inspire a love of the natural world, and our scientific expertise to inform solutions to the big, global challenges we face", he said.
The Duchess and Attenborough joined forces for the day to introduce the new blue whale skeleton, which measures 25.2m long and 4.5 tonnes and replaces the famed Dippy the dinosaur.
GETTYDuchess of Cambridge Kate sported her new shorter'do at the event
"And now, as a parent myself, I am experiencing the joy all over again with my own children, who adore coming here, and it is not just to see the T-Rex mind you". Kate took part in a special send-off for the exhibit previous year.
"'Putting our blue whale, Hope, at the centre of the Museum, between living species on the West and extinct species on the East, is a powerful reminder of the fragility of life and the responsibility we have towards our planet." he added.
Kate has been on a museum spree this summer.
Hintze Hall is due to reopen to the public on Friday following six months of refurbishment, and its displays will include a dinosaur fossil - an 120 million-year-old Mantellisaurus, found on the Isle of Wight in 1917.More news: Friday sale: Xiaomi Redmi 4A, Redmi 4, and Redmi Note 4