KSI to Justin Bieber, 'F*** You,' You Want Logan Paul To Rip My Head Off?

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.

"If anything, it's gonna be more funny when I knock out Logan," KSI says ... adding he's got some extra motivation now that the superstar singer is involved.

"I hope Justin Bieber's gonna be ringside, so after I knock out Logan, I can be like, 'Whose head was gonna get ripped off?!? You prick.'"

We also got Logan's take on Bieber's support ... and he says he's gonna obey Justin's instructions.

"He wants me to tear KSI's head off, I'mma f***ing do it."