It shows the Duchess of Cambridge looking relaxed in a grey wooly jumper and blue jeans.
"Thank you everyone for all your lovely messages on The Duchess of Cambridge’s birthday," the caption on the picture read.
The Duke and Duchess of Sussex replied: "Wishing a very happy birthday to The Duchess of Cambridge today!"
The Royal Family Instagram account, which is run by the Queen's staff at Buckingham Palace, posted a series of photos of Kate, as did Clarence House, run by Prince Charles and the Duchess of Cornwall's staff.
It comes after the Duke and Duchess of Sussex, Prince Harry and Meghan Markle, have caused shock across the world by announcing that they will quit the royal family and move to North America.
"We intend to step back as 'senior' members of the Royal Family and work to become financially independent, while continuing to fully support Her Majesty The Queen," the statement from the pair read.
"We now plan to balance our time between the United Kingdom and North America, continuing to honour our duty to The Queen, the Commonwealth, and our patronages. This geographic balance will enable us to raise our son with an appreciation for the royal tradition into which he was born, while also providing our family with the space to focus on the next chapter, including the launch of our new charitable entity."