Skate Star Ryan Sheckler -- Bieber's No Poser ... Dude Can Skate!

Justin Bieber's no joke on a skateboard ... the guy has REAL support from one of the best skaters on the planet -- Ryan Sheckler ... who tells TMZ Sports Justin's got real skills on wheels.

Sheckler was at LAX -- heading to Austin for the X Games -- when we showed him recent footage of Bieber riding one of those new motorized skateboards on his private jet.

Ryan LOVED IT -- and told us he thinks Bieber has the passion to take his skate game to the next level.

Sheckler would know ... the two actually skated together at Ryan's private skate park in Orange County back in 2014.

So, could we see Bieber competing in the X Games one day?

Sheckler isn't exactly ruling it out -- "Bieber can do whatever he wants man."