Marshawn Lynch -- Crackin' Jokes with Gronk ... On Conan O'Brien

Newsflash -- Marshawn Lynch has a FUNNY side ... and here's the hilarious video that proves it.

The Seattle Seahawks running back teamed up with the enemy (Rob Gronkowski) for a hilarious video game battle on "Conan" ... which quickly turned into a comedy session

The segment is pretty hilarious ... with Lynch crackin' drug jokes ... Gronk making a boner pill joke ... and overall hilarity ensuing.

Unclear when the guys found time to shoot the bit -- which is set to air Thursday on TBS -- but it seems like the two NFL superstars really hit it off.

#RealityShowSpinoff? #Please #BeastMeetsGronk #LethalWeapon5? #ImJustHereSoIDontGetFined