Rampage Jackson Says He'd Whoop Brock Lesnar's Ass

Rampage Jackson would kick the ever-living crap outta Brock Lesnar ... at least according to Rampage ... who says, huge weight difference or not, he'd win the fight.

TMZ Sports saw Jackson outside of 1 OAK in L.A. this week ... when a potential Brock Lesnar vs. Rampage Jackson super fight came up.

FYI -- Rampage is a legendary MMA fighter ... with a 37-11 record. Brock has a lot less experience (he's 6-3) ... but outweighs R.J. by 60+ pounds.

So, who'd win? Rampage has zero doubt he'd be victorious.

Question is, how would he take out Lesnar?