Police and paramedics were called to a commercial building undergoing renovations shortly before 4 a.m. Sunday.
According to witnesses, the victim was attending a private party on an upper floor of the building between Peel and Stanley streets. At that party, he became involved in a physical altercation.
Investigators questioned all of the 40 people still at the party when police arrived at the scene.
Early witness accounts given to the police suggest the man left the party after the fight but returned upstairs to find a piece of jewelry.
Const. Jean-Pierre Brabant of Montreal police said it's one of the theories investigators are looking at.
The man was found with what appeared to be stab wounds to his upper body.
Urgences-Santé paramedics tried to resuscitate the man, to no avail.
Ste-Catherine Street West between Stanley and Peel streets will remain closed into the late morning while police wrap up their investigation.