Toronto Animal Services and local volunteers are working to remove the cats and kittens and find them foster homes.
Neighbors had complained about a smell on the floor for months, according to Canadian media reports. Some neighbors knew the apartment had a lot of cats, but didn't suspect the number was that high.
The animals are being checked out for medical care, then spayed or neutered and vaccinated before they are put up for adoption.
This is the second major case of cat hoarding in that city this year. In March, more than 100 cats were discovered and removed from another home in Toronto.
"It's a terrible way for cats to live," Toronto Cat Rescue, a group that helped remove the felines, wrote on Facebook. "If you know that someone has too many cats, it is best to report it sooner rather than later. Unsterilized cats breed prolifically, and with only a nine week gestation things get out of control VERY quickly."