Norbert the Viral Therapy Dog Dead at 15

Norbert, best known as the viral high-fiving therapy dog, has died ... this according to his owner, Julie Steines.

Julie shared the somber update on Instagram Monday, informing fans "with a heavy heart" that the beloved pup -- full name Saint Norbert Liv Freyermuth Steines -- passed away in her arms Sunday night.

Per Julie, she and her family are heartbroken following Norbert's death ... which came 2 months shy of his 16th birthday.

She wrote ... "It was the honor of a lifetime to be entrusted to oversee his time here on earth. As a unique mixed-breed adopted dog— the only pup in his litter, he was a champion for pet adoption. Adopting him forever changed my life for the better in unimaginable ways."

Julie noted she planned to continue to post on Norbert's Instagram page in order to carry on his legacy as a social media star and a therapy dog ... which inspired a successful picture-book series and charitable endeavors.

While Julie didn't share a specific cause of death for Norbert in her tribute, her update comes days after the beloved pooch was "unexpectedly hospitalized for high kidney markers."

After some blood tests, Julie informed fans that Norbert's kidney markers were "still higher than we'd like," but noted he was still being sent home to receive ongoing care.

Hours later, Julie revealed Norbert had died at home.

Several of Norbert's followers and famous fans have sent Julie their condolences ... including "Glee" actress Dot-Marie Jones, the owners of Doug the Pug, and "Game of Thrones" star Nathalie Emmanuel, among others.