Shaquille O'Neal -- I'm Roasting Justin Bieber

He's burned Kobe ... He's burned Barkley ... and now Shaquille O'Neal has his sights on Justin Bieber -- 'cause he's been tapped to be an official flamethrower at the Comedy Central Roast.

Shaq -- along with stars like Snoop Dogg, Ludacris, Jeff Ross and Martha Stewart (what?!) -- will be tasked with ripping the Biebs to shreds in front of a packed house on March 30th.

Shaq and Bieber have history -- they once had a dance battle back before Bieber turned heel.