The Ottawa Humane Society says 12-year-old Napoleon was so heavy he could no longer stand or clean himself, while his fur became matted with fecal matter.
The group alleges Napoleon's owner had brought the cat to a veterinarian several times but failed to heed the animal doctor's advice, resulting in the pet's health deteriorating.
It says Napoleon was in severe pain when brought to the humane society and the odds of recovery were so poor the cat had to be euthanized.
Guylene Roy was charged last Wednesday under the province's animal cruelty law with permitting distress to an animal and failure to maintain standards of care, and faces a maximum fine of $1,000 and/or 30 days in jail.
She is due in court June 27.
OHS Inspector Miriam Smith says pet owners cannot ignore the instructions of their veterinarian.
"This cat was in very bad condition," Smith said in a news release.
"Failure to follow a veterinarian's advice and allowing a pet to deteriorate like this is appalling. And it's against the law."