Matt Barnes -- Reportedly Attacks Derek Fisher ... Are You Nailing My Ex?

NBA star Matt Barnes drove 95 miles from Santa Barbara to L.A. to pick a fight with Derek Fisher this weekend ... after he learned Fish has been dating his estranged wife ... so says the NY Post.

Barnes was reportedly in SB training with the Memphis Grizzlies when he heard the NY Knicks coach was in L.A. with his ex Gloria Govan. Barnes went nuclear -- hightailed it to L.A. and confronted Fisher in the backyard of the home.

According to the report, neither man suffered any major injuries. Cops were reportedly called to the scene but no one was arrested.

As for Fisher, he filed for divorce from his wife Candace back in March in one of the nastiest breakups in recent memory.