The candid snapshot shows the Duke and Prince George, then two, sat side-by-side on a carriage outside William and Kate's former home in Norfolk.
William's accompanied the picture with a heartfelt tribute to his granddad – and fans reacted in their droves to his poignant words, with many admitting they had been left in tears.
WATCH: Prince Charles delivers a moving tribute to his "dear Papa"
"Oh William, beautiful words," one follower wrote, with a second adding: "My heart. This brought a tear to my eye."
A third wrote: "The most beautiful message [crying emoji] and the most precious photo." And a fourth commented: "Brought tears to my eyes... thank you for this absolutely wonderful message, The Duke of Cambridge. Your grandfather was proud of both you and The Duchess of Cambridge and he's smiling at your lovely family from above."
Gwyneth Paltrow also commented on the post, simply writing: "This is beautiful."
Prince Philip passed away on 9 April, just two months before his 100th birthday. In his dedication, heartbroken William said: "My grandfather's century of life was defined by service – to his country and Commonwealth, to his wife and Queen, and to our family.
"I feel lucky to have not just had his example to guide me, but his enduring presence well into my own adult life – both through good times and the hardest days.
"I will always be grateful that my wife had so many years to get to know my grandfather and for the kindness he showed her. I will never take for granted the special memories my children will always have of their great-grandpa coming to collect them in his carriage and seeing for themselves his infectious sense of adventure as well as his mischievous sense of humour!
"My grandfather was an extraordinary man and part of an extraordinary generation. Catherine and I will continue to do what he would have wanted and will support The Queen in the years ahead. I will miss my Grandpa, but I know he would want us to get on with the job."
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