The 30-year-old wore a green silk Mulberry dress which she had previously worn at a reception at Buckingham Palace to launch the Queen's Jubilee celebrations.
Catherine, attending the official visit without Prince William, viewed an exhibit alongside director of the Natural History Museum Doctor Michael Dixon and special collections manager Judith Magee, before giving a brief speech at the event.
The Duchess said: "The Natural History Museum has a very special place in the heart of this nation. William and I are just two of millions of people who have passed through these doors, and marvelled at the spectacular wonders of the natural world, housed in this beautiful gallery."
Have a look at the Duchess's style over the past year here: