Jay Z -- I'm In the Fightin' Business ... Signs Top Boxer to Roc Nation

Baseball? Check. Football? Check. Basketball? Check ... and now Jay Z can add boxing to the list of sports in his Roc Nation sports empire -- inking super middleweight champ Andre Ward.

Ward -- who's 27-0 and recognized as one of the best pound-for-pound fighters in the world -- announced this morning, "It's official!! #NextChapter Happy to be a part of the @rocnation family."

It seems like a good union for both parties ... Jay's reportedly been trying to get into the boxing game for a while but was holding out for a fighter with real star-power.

As for Ward -- he hasn't fought in nearly two years due to business disputes with his former promoter ... but it's clear he feels Jay can get him back on track.

Plus, anything that brings you closer to Rihanna has got to be a positive.