The photograph, taken in the last few days by her mother, the Princess of Wales, shows Charlotte, the couple’s second child, smiling happily in the garden of their home in Windsor.
In a post on X, the couple said, “Happy 9th Birthday, Princess Charlotte! Thank you for all of the kind messages today.”
Kensington Palace has established a tradition of releasing photographs taken by Kate to commemorate birthdays and other family events.
However, a picture issued earlier this year on Mother’s Day in March was retracted by several news organisations due to editing, something for which Kate later apologised.
This incident happened shortly before the princess disclosed that she was undergoing preventive chemotherapy after tests following major abdominal surgery in January that revealed the presence of cancer.
The heir to the throne, William, resumed official duties last month, but Kate will only return to the public eye when her medical team confirms she is well enough. King Charles returned to public engagements this week for the first time since being diagnosed with cancer in February.
Last month, the couple also released a photograph of their youngest child, Prince Louis, to mark his sixth birthday. The image taken by Kate, showed Louis lying on his stomach on the grass in a close-up shot.
(With inputs from agencies)