EXCLUSIVE

Starlin Castro is gonna have some seriously filthy fun before baseball starts up again -- the Chicago Cubs shortstop just scooped up a new off-road toy, and turned it into a rolling nightclub.

TMZ Sports has learned the 3-time All-Star bought a 2015 Polaris 1000 -- a serious all-terrain vehicle -- which he pimped out with 4 custom 24-inch waterproof speakers.

The Auto Firm in Miami did the tricking out, and owner Alex Vega tells us Starlin shelled out $25K for the customizing, on top of the $20K for the ATV.

Maybe the pimpest addition -- the speakers AND custom wheels light up in different colors! They'll see and hear Starlin coming from blocks away ... when he ships the rave-on-wheels down to his native Dominican Republic.