Justin Bieber -- High? Or Definitely High?

Justin Bieber just posted a video on Instagram ... and let's just say he appears a bit elevated.

Bieber -- with his shirt off and a chain around his neck -- is trying to deliver a message to his fans about his excitement over the new ability to post video on Instagram ... but can't get through 3 words without cracking up.

Now, we're no weed experts ... (ok, some of us are) ... but it seems like Bieber's next move was probably a munchie run.