Charles, 75, and Camilla, 76, mingled with 500 members of the Diplomatic Corps in the state rooms at Buckingham palace on Tuesday evening.
Buckingham Palace shared an official portrait of the foursome in the palace's 1844 Room.
The King wore a tuxedo while his wife looked glamorous in a cream embroidered evening gown by Fiona Claire and jewels from the late Queen Elizabeth II's collection.
For the second time this year, Camilla sported the Girls of Great Britain and Ireland Tiara along with the late Queen's diamond bracelet and the late Queen Mother's diamond brooch.
Kate, 41, looked stunning in a shimmering pink Jenny Packham gown, previously worn at the Jordan royal wedding in June, which she accessorised with her go-to jewels, the Lover's Knot tiara.
The Princess also wore the late Queen's Greville Chandelier earrings - the diamond jewels were given to Princess Elizabeth by her parents as a wedding gift in 1947.
William donned a tuxedo and like the King and Queen, displayed his Garter star and sash.
The Diplomatic Reception takes place annually at Buckingham Palace, with Charles hosting for the first time last year, just three months into his reign.
Earlier in the day, Kate opened a new children's surgery unit at Evelina London Children's Hospital in London.
The new £55 million children’s day surgery unit enables staff to provide a one-stop service for youngsters needing surgery.
Chatting to one family in a pre-op cubicle, Kate said: "It’s always nerve-wracking as a parent; I've taken mine into hospital for different things. You're always at least worried but you've got a great team (here)."
William was also spotted as he was reunited with Big Issue seller Dave Martin to sell copies in London.
Kensington Palace later shared an image of the outing on social media.
Meanwhile, the King was given a handwritten Christmas letter by a young girl as he visited Coptic Orthodox Church Centre UK in Stevenage, Hertfordshire.
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