It's unclear why Lupo passed on, although it's obvious he enjoyed every day of his life with his regal dog parents. The couple regularly shared pictures of their fur-baby on Instagram, including an adorable snap of Lupo sharing an icecream with his human brother Prince George.
A touching tribute
The couple also used Instagram to share the tragic news, posting a beautiful picture of Lupo and this touching tribute:
"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."
The picture received almost 900k likes and thousands of messages of condolence from Kate and William's fans.
Dogs are part of the family
"Oh, I remember when Lupo was a pup! Those furry family members mean so much and I'm so sad to hear of your loss," posted one person.
"Losing a beloved family dog is so hard for all," wrote another. "But Remember the beautiful life you gave him, and the one he helped give you."
Sleep well, little Lupo. Gone, but never forgotten.