Tyson Chandler -- Justin Bieber Is My Homeboy

Justin Bieber has some big friends ... we're talkin' 7-footers ... and they've got JB's back no matter what, so says NBA superstar (and full on Belieber) Tyson Chandler.

Earlier this week, Bieber tweeted a photo of himself (sans shirt, of course) hanging out with Chandler and Oklahoma City Thunder superstar Kevin Durant.

Everyone sorta wondered how they all know each other ... and what they were all doing at Bieber's mansion.

Well, last night, we found 7'1" Chandler leaving a party in Beverly Hills and he told us everything ... except why Bieber has so much trouble keeping his shirt on.

The mystery continues ...