Jorge Masvidal was down bad after Dustin Poirier got knocked out at UFC 291 ... not only because he hated seeing his friend lose, but he also kissed a whopping $100k goodbye after betting on the fight.

The retired UFC star revealed his woeful wager shortly after his American Top Team teammate lost to Justin Gaethje via a vicious leg kick during Saturday's BMF belt matchup ... admitting he was so confident "Diamond" would come out on top, he was willing to risk a hefty chunk of change.

FYI -- Poirier was a -150 favorite to beat Gaethje, which means Jorge would have pocketed an extra $66k had it gone the other way.

Gamebred told TNT Sports he "probably shouldn't" be making bets of that magnitude ... and joked his three children will suffer from the financial blow.

Of course, Jorge will be okay -- he previously said in an interview he once made $5 million off one fight alone ... and he has his own Gamebred Boxing and Bareknuckle promotions that are taking off in his retirement.

As for the fight, Jorge felt Dustin had the upper hand in the first round ... but Gaethje had the "perfect kick" in the second that knocked him out.

"So hats off to Justin, man," the former BMF title holder said. "Great fight, great competitor always."