William recorded his personal recollections for the digital memorial in honour of Elizabeth II.
Memories of the nation's longest-reigning monarch by members of the public and those who knew her are now live and can be viewed at queenelizabeth.com.
The prince, in his own contribution to the digital archive, recalls spending "many a quiet afternoon" with the Queen and the late Prince Philip, Duke of Edinburgh at Windsor Castle, where his grandmother was "completely at ease" surrounded by her family, dogs and horses.
William described how the Queen would ride her horses in Windsor Great Park, even in her later years, showing a "side of Granny the world didn't often see - off duty, relaxed and utterly at home".
The prince, who recorded his video at Windsor, said: "Whilst I had the privilege to know the late Queen better than most, some of my fondest memories of my grandmother are from here in Windsor.
"I remember many a quiet afternoon having tea with her and my grandfather at the castle, chatting, sharing stories, and seeing how completely at ease she was here, surrounded by family and her much-loved dogs and horses.
"To this day, she's given me a love of teatime that I never knew I needed.
"Watching her ride her horses in the Great Park, even well into her later years, showed a side of Granny the world didn't often see - off duty, relaxed and utterly at home.
"These moments with my grandmother are ones I'll always hold dear."
The Queen would take afternoon tea daily at around 5pm, and it would usually consist of two types of sandwiches, as well as scones, small cakes and Earl Grey tea.
The Queen Elizabeth Digital Memorial was launched on what would have been the Queen's 100th birthday in April.
The Court Circular, the official daily record of royal engagements, from throughout the Queen's reign has also been digitised, giving a searchable record of her public appearances and duties across seven decades.
Members of the public are invited to share their own memories of the Queen at queenelizabeth.com.
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