A photo shared to the couple's Royal Kensington account on Instagram showed the black cocker spaniel sitting down and looking into the distance.
The lovely picture was captioned with the heartbreaking news.
It read: "Very sadly last weekend our dear dog, Lupo, passed away. He has been at the heart of our family for the past nine years and we will miss him so much. - W & C."
Royal fans were quick to share their sympathy with the couple. One commented: "I am so so sorry!! Losing a dog is so painful. Love to you all."
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Others added: "I'm so sorry, it is so difficult to lose a beloved pet," "So sad to hear," and: "Oh nooo. Rest in peace Lupo."
Lupo had a family connection as he was bred from Ella, a dog owned by the Duchess' parents, Michael and Carole Middleton.
When he was just a puppy, Kate took the pup to a polo match, where he greeted Prince William and Prince Harry.
Kate could often be spotted with Lupo at events
Over the years, he has often been spotted walking with the royal couple, including on the beach, and even inspired his own series of books for children, The Adventures of Lupo.
Kate's brother, James Middleton, who owns five dogs himself, also paid a heartfelt tribute to Lupo on Instagram.
His message read, in part: "Nothing can ever prepare you for the loss of a dog. For those who have never had a dog, it might be hard to understand the loss.
The couple announced the sad news on Instagram
"However for those who have loved a dog know the truth: a dog is not just a pet; it is a member of the family, a best friend, a loyal companion, a teacher and a therapist."
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