The 53-year-old legend just dropped some brand new training footage after that 5-second teaser last week ... and it's scary good.

After throwing multiple ferocious combos -- and putting his trainer through hell -- Iron Mike looks at the camera and declares, "I'm back."

It's the same two words Evander Holyfield says in his comeback video, which he just dropped last week.

As we previously reported, both Mike and Holyfield are interested in getting back in the ring to raise money for charities through exhibition fights.

We spoke with Holyfield about a possible 3rd clash with Tyson -- the man who bit off his ear in 1997 -- and he told us he's down.

Now, the question is ... when can we get this 3rd fight going -- and how much money his at stake?!

We know Tyson has been getting multimillion-dollar offers from multiple organizations and promoters ... but ya gotta think the biggest payday could come by way of a Tyson vs. Holyfield III.

For the record, Holyfield won BOTH of their previous fights -- but the way Tyson looks now ... yeesh!!!