Floyd Mayweather's next opponent is the total package ... in more ways than one ... and last week, he let it all hang out in a SUPER-REVEALING underwear photo shoot ... and TMZ has the pics.

WBC World Welterweight Champion Victor Ortiz is now the spokesman for an underwear site called Freshpair.com -- and as part of the deal, the 24-year-old agreed to model some of his favorite undies.