"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 couple wrote in an Instagram post Sunday.
The message, which included a photo of their pet in happier days, has gotten more than 500,000 "likes."
Lupo was an English Cocker Spaniel, who was 9 years old. William and Kate welcomed Lupo in December 2011 about a year and half before the birth of their first son, Prince George.
William, 38, and Kate, 38, married in 2011. They have three children -- George, 7; Princess Charlotte, 5; and Prince Louis, 2.