Dr. Robert Rey didn't just punch the guy who grabbed his balls -- he unleashed the full fury of his 2nd degree black belt ... and says he has no regrets about it.
The former "Dr. 90210" star came on TMZ Live ... and said this wasn't the first time he's been attacked this way -- in fact, he says men in Brazil frequently call him gay and grab his junk, or kiss, or blow in his ear. He says this weekend's incident was just the final straw.
Watch the video ... Dr. Rey breaks down the moves he used to subdue nut-grabber.
The lesson here -- keep your hands off the doc's jewels. Or else ...