Case in point ... here's Daymond John outing one of his fellow "Shark Tank" stars whose credit card was DECLINED at a fancy dinner -- and all signs point to MARK CUBAN!!!!

"A card was declined and it was somebody that is the last person that I ever think a card would be declined," Daymond said on Rich Eisen's new "Just Getting Started" podcast.

"And there was obviously a reason ... but I was like, '[clears throat] I’m going to get this one, so that I can go down in history saying that I paid for your check!'"

Then Daymond dropped another clue ...

"And, if you knew how the Sharks talk, you can tell -- if you’re in sports -- he said 'Whatever, DJ,' you know who that is, of course."

IT'S DEFINITELY MARK CUBAN!!!

Of course, Cuban is currently worth around $4.4 BILLION, according to Forbes ... so the declined card was most likely a mistake (it's not like the dude doesn't have the funds)!

But still, Daymond loved EVERY SECOND OF IT -- and now he can brag that he picked up Cuban's check in the Dallas Mavericks owner's moment of need.

At the end of the interview, Eisen tells Daymond -- "You tell Cuban, pay his bills!"

Daymond smiles and replies, "You said it, not I. I didn’t say the name, you said it."