Emergency responders were called to an apartment in the Gay Village on Ste-Catherine Street near the corner of Montcalm Street to treat a man in cardiac arrest.
A 34-year-old man was found dead at the scene.
Montreal police Const. Manuel Couture said the man had no wounds on his body, but there was blood on the scene.
"Some elements on the scene indicate it could be a suspicious death," Couture said.
A 30-year-old man considered an important witness in the case will be questioned by police today.
The street has been closed by police and the major crimes unit is investigating.