The photograph was taken in May and features Charles and Camilla in the morning room of their London home, Clarence House.
Camilla wears a navy coat by Fiona Clare, accessorised with a pearl necklace and earrings, while Charles dons a navy British wool suit with a light blue tie.
Peruvian photographer Testino said he was "honoured" to capture the pair on such a special date, releasing the statement (via BBC News): "I first photographed the Duchess after her wedding to the Prince on a commission from British Vogue in 2006, and discovered a kind and beautiful person with a wonderful sense of humour.
"Doing this latest assignment to celebrate Her Royal Highness' 70th birthday is a true honour."
A spokesperson for Charles and Camilla added: "The Duchess was photographed with the Prince at home at Clarence House by world renowned photographer Mario Testino who has photographed them both several times before.
"The image is one of several that was taken that day. Although these were not shot as official birthday portraits their Royal Highnesses liked them very much and felt it would be nice to share them to mark the day."
Testino is a firm favourite with the royal family, and previously photographed Prince William and Kate Middleton for their engagement portrait in 2010, and taken photos of Prince George and Princess Charlotte.
He also captured a series of portraits of the late Diana, Princess of Wales, just a few months before her death in 1997.
Meanwhile, Camilla celebrated her birthday with a party over the weekend at the couple's home, Highgrove House, in Gloucestershire.