Justin Bieber -- I'm on a Boat ... But It Isn't Mine!

Justin Bieber is making a habit of making it look like he owns cool stuff, when he really doesn't -- this time he tried to pass off a super famous yacht as his own ... when everyone knows it's not his.

As TMZ first reported, Bieber posted a photo of himself in a Bugatti that he referred to as "my first Bugatti" ... but it was just a loaner from Birdman.

Then on Wednesday, Bieber posted a pic on board a yacht called "Never Say Never" -- which just happens to be the name of his concert film. But TMZ has learned Bieber does NOT own the boat ... not even close. The boat is registered to a family trust based in Coral Cables, FL.

Here's why Justin is particularly stupid for trying to lay claim to the yacht -- it's the same boat Lonely Island used for their LEGENDARY video, "I'm on a Boat."

If you want to make it in the rap game, Justin ... might want to start getting your own stuff instead of borrowing other people's.