Shaq paid off some young guy's engagement ring debt at a jewelry store in Georgia this week -- and the amazing gesture was all caught on video.

The Big Diesel explained how it all went down on "NBA on TNT" on Tuesday night ... saying he was in a Zales in the Atlanta area on Monday shopping for some earrings when he noticed a man was stressing over a bill.

Shaq says the guy was trying to figure out a way to set up a payment plan for a ring he was purchasing for his girlfriend ... when the NBA legend felt like he just had to step in.

"I seen the guy come in -- he was just so shy," Shaq said. "He was saying, 'Hey, how much do I owe to pay off my ring.'"

"And I was like, 'My man, how much is the ring?' I'm not going to say the amount, but it's not a lot for me."

So, Shaq whipped out his credit card -- and paid for it right on the spot!!

"I was like, 'You know what? Tell your girlfriend I got her,'" Shaq said. "'I'll take care of it.'"

Shaq added, "At first, he didn't want to take it. He was like, 'Man, I can't do that.' I said, 'Don't worry about it. I do it all the time.' I'm just trying to make people smile, that's all."

The two then posed for a photo -- and it was all really freakin' awesome!!

The best part of the story, though? Shaq says he had an even MORE generous act at a furniture store recently, paying off a bill there for a sweet family.

"This lady, she had an autistic daughter," Shaq said. "And she was also looking to pay for some furniture and I just took care of her. So, I'm into making people happy."

"Whenever I leave the house, I just try to do a good deed."