William and Kate marked the milestone with the release of a set of latest pictures of the infant prince.
"We would like to take this opportunity on George's first birthday to thank everyone over the last year, wherever we have met them, both at home and overseas, for their warm and generous good wishes to George and our family," the Duke and Duchess of Cambridge said in a statement.
They are preparing for a party at their Kensington Palace apartment, which is expected to be attended by his great-grandmother Queen Elizabeth II and aunt and godmother Zara Phillips and her six-month-old baby Mia.
Prince Charles and the Duchess of Cornwall are not able to attend as they are in Scotland.
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Prince George was born on July 22 last year and first presented to the public on the steps of St Mary's hospital in London on July 23, 2013.
He has since made fans around the world, when he travelled with his parents to New Zealand and Australia on a royal visit.
A new set of pictures of the toddler prince were published last night.
One of the images shows the young royal fascinated by a blue morpho butterfly which has landed on his father's hand.
The other image shows the two future kings and the Duchess of Cambridge looking at an object above their heads.
The pictures were taken a few weeks ago at London's Natural History Museum.
Photographer John Stillwell said, "George was very lively and very sure of himself and confident - a very determined young boy".