If Floyd Mayweather fights in the UFC, Demetrious Johnson says he knows exactly how it'd go down -- "He'd get his ass whooped."

TMZ Sports spoke with the UFC's pound-for-pound king in Minneapolis ... and he laughed off the idea of Floyd competing in MMA -- even though there are signs the boxer might be legitimately interested. 

But whether it's against him, Conor McGregor or any other UFC fighter ... Demetrious says Floyd can't hang, and "he knows it."

If Mayweather DID make the jump ... DJ says he'd have no problem showing TBE how much better MMA is than boxing -- by trying to SNAP HIS ARM.

Your move, Floyd.