Kate viewed the latest exhibitions and met guests at the 2017 Portrait Gala, before having dinner at the London attraction.
The Duchess, who has been patron of the gallery since 2012, dazzled in a full length, lace green gown by Temperley.
She wore her hair loose and carried a gold sparkly clutch, finishing off her look with glittering drop earrings.
Among those she met as she made her way through the gallery were fashion designer Erdem and fashionista Alexa Chung who is chair of the 2017 gala committee.
She viewed two exhibitions - Howard Hodgkin: Absent Friends, and Gillian Wearing and Claude Cahun: Behind The Mask, Another Mask.
To help raise funds, a range of artwork has been especially created, including 10 masks by the likes of Dame Vivienne Westwood and Philip Treacy, and 100 postcard-sized works of art for a mystery portrait postcard sale.
Kate had the chance to view both of these collections during the evening and said of the postcards: "Well, they're all amazing."
This is the second gala Kate has attended and, as in previous years, the funds raised will support the National Portrait Gallery's work in delivering exhibitions and displays, offering learning opportunities and undertaking research.
The 2017 gala is fundraising for Coming Home, a project that will make it possible for portraits of people to return to places that are special to them for a loan period of over three years.
Coming Home will enable sitters such as Sir Walter Raleigh to return to Dorset, the Bronte sisters to Yorkshire and David Beckham to Essex.
Earlier, it was announced Kate will visit Luxembourg to attend commemorations marking the 150th anniversary of a treaty that confirmed the country's independence and neutrality.
She will travel to the country on May 11 to attend events celebrating the 1867 Treaty of London, at the request of the Foreign and Commonwealth Office.