The young prince - who is the oldest child of Prince William and Catherine, Princess of Wales - posed in a dark suit, with his white shirt unbuttoned at the collar and without a tie, in the image, which was taken by photographer Matt Porteous at Kensington Palace after Trooping the Colour in June.
The image was shared on William and Catherine's social media channels along with a caption that featured a party popper emoji and the words: "Happy 13th birthday, George!"
This is set to be a big year for George, who is second in line to the throne, because he'll be starting a new school in September.
It was recently confirmed the prince's destination after Lambrook - where his siblings Princess Charlotte, 11, and eight-year-old Prince Louis are pupils - will be Eton College, the prestigious all-boys boarding school that was attended by his father William and uncle Prince Harry.
A royal spokesperson said in June: "Kensington Palace can confirm that Prince George will attend Eton College from this September."
The school - which costs around £63,000 ($A120,000) per year - is close to the family home in Windsor, but is full boarding, though there is flexibility for pupils to go home at weekends.
Like William and Harry, George will board at Manor House, where it is expected rooms will be provided for his protection officers, as was the case for the older princes and their security teams.
William's own attendance at Eton broke with royal tradition because his father, King Charles, grandfather Prince Philip and uncles Prince Edward and Andrew Mountbatten-Windsor all attended Gordonstoun in Scotland.
Meanwhile, Harry - who is estranged from his father and brother - described Eton in his memoir Spare as both the "finest school in the world" and a "profound shock".
He added: "It could only be purgatory for one very unbrilliant boy".
William and his wife Catherine, Princess of Wales, were said to have considered other schools for George, and at one point, it was reported they were keen to send him to Marlborough College in Wiltshire, a prestigious co-ed boarding school attended by the princess between the ages of 14 and 18.
A source told Vanity Fair in January: "While Eton would be the obvious choice because it is on their doorstep and where William went, they are leaning towards other schools. I'm hearing Marlborough is the front-runner.
"I genuinely don't think they've one hundred per cent decided, but there is talk among the Marlborough set that they've allocated a house for George and that security has been a big issue."
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