The Santa Fe Animal Shelter of New Mexico said Meow, who weighed 40 pounds, died of pulmonary failure.
The two-year-old cat had been surrendered to the shelter as his 87-year-old owners could no longer take care of him.
Employees at the shelter had been working to put Meow, who weighed the equivalent of a 700 pound, on a diet so he could shed weight and be adopted.
Four different veterinarians fought to save the cat’s life to no avail, the shelter’s Mary Martin said on Facebook.
‘The shelter staff – along with all those who met Meow during his short time with us – mourn his passing,’ she said in a statement.
‘Meow had been doing so well in his foster home; walking up stairs and seeking affection – that it is so very hard to believe he is gone.’
She added: ‘We will forever be grateful for the attention Meow’s size brought to pet obesity and to animal shelters across the country. We are especially grateful to all of you who fell in love with this charming cat – as we did – and were so very interested in his progress and success.’
The shelter had previously described Meow as ‘very sweet’ but warned he faced similar health risks to those experienced by a ‘morbidly obese person’.
‘We got him a scratching post with a carpeted ring attached but he couldn’t even get his head through,’ spokesman Ben Swan said.
‘He had no interest in the super-sized toy mouse we gave him either.’
The fattest cat in history recorded by Guinness World Records is Himmy from Australia, who weighed 35 pounds.
But the category was later scrapped amid concerns it would encourage over-feeding of pets.