Investigators say a neighbour called officers to the building on Manning Street near LaSalle Boulevard at around 6:30 p.m. Monday.
Raphael Bergeron, a spokesman for police, said the victim was found with serious injuries to his upper body and later pronounced dead at the scene.
Bergeron said the man was known to police.
No arrests have been made in the case and officers were still at the scene early Tuesday.
It was the fifth homicide in Montreal this year.