JB responded to an IG post Shawn threw up on Sunday -- a magazine cover of himself with the headline, "Prince of pop." Biebs told his fellow Canadian pop star pal to pump the brakes ... 'cause Justin insists he's still the chosen one of the genre.

According to Bieber, Shawn has a long way to go and a lot more records to break before he can touch the crown. He didn't seem malicious about it though, describing his ribbing as being delivered in a "Canadian voice" -- something they'd both understand.

Justin has an interesting proposition for Shawn though, telling him if he wants ... they could decide this Prince of Pop thing once and for all over a game of hockey.

It sounds like Justin's taking even more shots, 'cause he says he's heard Shawn's a "bender" on the ice ... meaning he can't skate. And, of course, we know Justin can.

FWIW ... it's understandable why Shawn would think he's dethroned Justin. Dude hasn't released very much new solo music since 2015 ... and Shawn's been grinding nonstop for a while now.

Granted, JB's been focusing on his personal life and well-being -- and ya can't knock him for that -- but someone was bound to step into the PoP convo eventually.