Another victim, a 21-year-old woman, suffered less serious injuries and is recovering in hospital. Police said she may have been hit by a ricocheting bullet.
Police received a 911 call around 2:40 a.m. ET reporting gunshots. They arrested 25-year-old man outside the venue shortly after the shooting.
The major crimes unit is speaking with the many witnesses who were inside the venue at the time.
Meanwhile, police cordoned off the street in front of the Cabaret du Mile-End venue and have closed the street between Fairmount and St-Viateur to both vehicle and pedestrian traffic.
The shooting is the ninth homicide of the 2014.