Justin Bieber -- I Can't Party Without A $20k Skate Ramp (PHOTOS)

Justin Bieber better bring his best ollies and stick those 360s ... because a Montreal club has built a very unique $20,000 skate ramp for his one-off appearance.

Bieber's hosting a party Saturday at Beachclub -- the same place that threw down for Kylie Jenner's 18th -- and the place has been hammering away for weeks on a custom mini-ramp where Bieber and his guests will shred.

If you're wondering why a wooden skate ramp costs $20k ... it's because it freakin' floats! We're told $10,000 alone got sunk into the ramp's floating supports.

Bieber will also get to hit up the club's adjacent wakeboarding park. The club's getting something back for all its trouble -- we're told Justin's throwing in a 3-song performance to say thanks for the extreme party.

Skate or die!