Justin Bieber is siding with Logan Paul in his boxing rematch with KSI next month ... and guess who ain't happy about that???
Yup ... KSI tells TMZ Sports he has one message for Bieber -- "F*** you, Justin!!!"
The Biebs made his Team Paul allegiances known in the IG comments when the Nov. 9 fight at Staples Center was announced last month ... telling Logan to "rip his head off."
KSI admits it's pretty cool to know that Bieber knows who he is, but makes it VERY clear he's gonna prove JB backed the wrong boxer.