Lupo was bred from a dog owned by Kate's parents Michael and Carole Middleton, and given to the couple as an early Christmas present in 2011.
"Very sadly last weekend our dear dog, Lupo, passed away," William and Kate wrote alongside a photo of their beloved pup. "He has been at the heart of our family for the past nine years and we will miss him so much. - W & C."
Lupo was loved by his family, and made several appearances in official portraits over the years, including just following Prince George's birth.
The dog also helped William and Kate choose George's name. The couple said they scattered pieces of paper with various names on the floor. Lupo stopped when he reached "George."
Kate's brother, James Middleton, mourned Lupo's death on Instagram.
"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. 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," he wrote.
"There isn’t much of a rule book on how to grieve for a dog, but I’ve said a prayer, lit a candle and taken Ella (Mum) for a long walk to spend time remembering Lupo," James added. "Rest in peace Lupo. Tilly & Mini will be waiting for you. You will always be remembered and your legacy will live on forever. Good Boy 🐾."
See more on the family in the video below.