Stone Cold Steve Austin -- I'm Gonna Kick Justin Bieber's Ass ... If He Doesn't Straighten Up

Stone Cold Steve Austin just threatened to open a can of WHOOP-ASS all over Justin Bieber ... warning the pop star that if he doesn't straighten up, he's gonna catch a Size 13 in his ass.

The former WWE champ was on "The Arsenio Hall Show" last night when he issued the ultimatum -- shape up ... or else.

Stone Cold says he loves kids ... and just wants to make sure Justin gets on the right path before it's too late -- but he thinks an ass-kickin' is the only thing that'll get through to the Biebs.