Karl-Anthony Towns is going OFF on all the trolls talking down on his GF Jordyn Woods' recent body transformation ... saying he's personally seen her put in the work -- and it's ALL natural.

Woods was the center of awful social media criticism this week after posting a before-and-after pic from her fitness journey ... with some claiming her new look was manufactured.

But, KAT is having none of that talk ... shooting down any notion Woods went under the knife in a passionate Twitter rant on Thursday.

"Are we saying that women can’t work hard and transform their bodies??" Towns said minutes ago. "I’ve SEEN the hard work and I’ve seen the results and trust me, this is all natural."

"Since she was 10 years old, people been calling this woman vile things because of her weight and instead of letting the Internet run her life, she went out the put the damn work in and changed her whole life. FITNESS SAVED HER LIFE!"