A Kensington Palace spokesman said the Duke and Duchess were “delighted” to share the new image to mark George’s birthday.
“The photograph was taken while on holiday in the UK earlier this month by the Duchess,” they said.
The portrait is the second taken by the Duchess to be published within a month, after the Duchess of Cornwall commissioned her to take her picture for the cover of Country Life magazine.
A keen amateur photographer, she regularly releases pictures she has taken of her children to mark their birthdays and is patron of the Royal Photographic Society.
The Cambridges are understood to have headed off for a summer break shortly after the children broke up from school.
On their return, they begin a new chapter with a move to Windsor and a new school for all three children to start together in September.
For the first time, Prince Louis, four, will join Prince George, nine and Princess Charlotte, seven, as they don their uniforms ready to embark on a new school year.
Relocating to Adelaide Cottage
The family will relocate from Kensington Palace to Adelaide Cottage, a four-bedroomed property just a stone’s throw from Windsor Castle.
The move will take them closer to the Duchess’s family in Bucklebury, Berkshire, as well as being within walking distance of the Queen, 96.
It will allow the Duke to keep in close contact with his grandmother and her staff, who are largely based at the castle.
The vast Windsor estate will give the children more scope to play, ride horses and walk their dog in privacy.
The Duke and Duchess will also be on hand to do the school run and immerse themselves in busy prep school life, with its many sporting and social activities.
The young royal and future king turns nine on Friday and, according to reports, will start at Lambrook School, in Ascot, near Windsor Castle, later this year.