Nate Diaz says he was promised $9 million for his fight with Jorge Masvidal ... but he claims he never got the cash -- and now, he's suing to get money he says he rightly deserves!
The former UFC star filed the lawsuit in U.S. District court in Miami on Monday ... less than two weeks after he and Masvidal squared off in a boxing match at the Honda Center in Anaheim.
Diaz claims in the suit, obtained by TMZ Sports, that before he ever stepped in the ring with Game Bred .