Justin Bieber -- Pelted By Water Bottle in Brazil

Justin Bieber took a water bottle to the face during a live concert in Brazil this weekend ... just like he did in 2009 ... but this time, he reacted MUCH, MUCH differently.

You remember the 2009 shot, when someone beaned the cute, adorable (shirted) pop star -- and he reacted by smiling and telling the crowd, "Ow! That didn't feel too good."

But Brazil was different -- an unshirted Bieber got smacked (while singing "Boyfriend") ... and reacts by mad-doggin' the crowd and storming off the stage.

How quickly they grow up ...

9:30 AM PT: Looks like Bieber had quite the weekend in Brazil -- during a show the next night a fan bum-rushed the stage ... and was swiftly taken care of by the backup dancers.