NY Knicks rising star Dennis Smith Jr. did ... revealing the key to his Summer weight loss was cutting out the fried deliciousness from his favorite fast food spot on the planet, Bojangles!!!

For those who aren't familiar with the chicken 'n’ biscuit chain, it's HUGE in southeastern states like North Carolina (where Smith Jr. is from). Cam Newton's raved about it too!!!

So, when the 21-year-old showed up to training camp looking slimmer and trimmer than usual ... the media pressed him to reveal his weight-loss secret.

"Yeah, I had to change some of the things I ate," Smith Jr. said, "I’m a country boy, man. We eat DIFFERENT down there."

"I ain’t eat Bojangles for most of the summer ... it helped!"

Of course, Smith says he also ate a bunch of green stuff like kale and spinach ... but it SUCKS compared to Bojangles.

Smith says he feels lighter and stronger and thinks the weight loss will help him better endure the long NBA season. He's probably right.

Also worth noting ... there are NO Bojangles franchises in New York -- so he's out of the danger zone during home games.

But, NY plays on the road in Charlotte, D.C. and Atlanta ... where there are a TON!!!