"Rest in peace, my beautiful Diesel," she wrote Sunday on Twitter. "I hope you're playing Frisbee at the dog park in the sky. I love you forever."
The dog's health became a big concern for Milano in January.
"My dog Diesel is sick. They think it could be liver failure. Please keep him in your prayers. (Thank you.)" she shared Jan. 12 on Twitter.
But a few weeks later, Milano discovered that Diesel's condition was very serious – and she was devastated by the news.
"My Diesel has cancer," she wrote. "I'm heartbroken."
The mom of a son – 17-month-old Milo – has a brood of other animals to comfort her during this difficult time, including 10 horses and pups Dodger, Lucy, Gibson and Quixie.