The legendary boxer flaunted his bod in a topless workout vid Wednesday for the Chinese social media site Weibo ... and the 53-year-old is jacked!!!

Iron Mike showed off a set of six-pack abs, some chiseled pecs and some pumped-up biceps ... and, good lord!!!

Oh, and by the way, he also flashed his scary punching speed and power!!

The video is another clear attempt to gas up an upcoming charity fight that Mike's trying to set up (possibly with Evander Holyfield) ... but the body he's showing off is seriously wild.

What's even more impressive about this video is that Tyson had gained a significant amount of weight a few years back -- and just last year Tyson revealed he had stopped working out altogether.

So, the fact he could bounce back into this kind of shape while in his 50s -- it's pretty damn incredible.

Good luck to whoever agrees to that charity fight!!