Boo, the 12-year-old Pomeranian pooch often dubbed the "world's cutest dog," has died, his owners reported on social media Friday.

"Boo passed away in his sleep early this morning and has left us to join his best friend, Buddy," the post says. "Our family is heartbroken, but we find comfort knowing that he is no longer in any pain or discomfort."

Buddy was Boo's canine chum who passed away in 2017.

"Shortly after Buddy died, Boo showed signs of heart issues," the post read. "We think his heart literally broke when Buddy left us."

More than 16 million people followed Boo on Facebook, and more than 560,000 followed him on Instagram.

Boo was the hero of a book called Boo: The Adventures of the Cutest Dog in the World. He also appeared on national talk shows, was the subject of a calendar, and the model for a stuffed toy.

On the announcement post, fans shared what Boo had meant to them.

"I would like to thank Boo for being my center of focus for the last 7 years fighting cancer," wrote Sarah Gruarin. "You see, my sister went to the gift shop in the hospital and bought me a tiny Boo stuffed animal. Every time I'm in surgery Boo comes with me and is my friend. He also inspired me to get a beautiful black Pomeranian named Little Luigi who now is my service dog."