Justin Bieber might need a lesson in "know your audience" ... 'cause fans booed the hell out of the singer during his tour stop in Montreal on Tuesday for dissing the hometown Canadiens.
Bieber -- a MASSIVE fan of the Habs' hated rivals, the Toronto Maple Leafs -- showed love for his team right before his final song of the night ... saying, "How about those Leafs though, huh?"
The "Ghost" crooner even gave a shoutout to his close buddy for scoring a goal against the Canadiens on Saturday .