It's true ... he hadn't joined the 500-pound bench press club -- but he changed all that with one hulk-like lift Tuesday ... and yeah, it was impressive as hell.

The ex-Steelers superstar loaded up 10 plates and then some on his bar to get up to the half-a-thousand-pound mark ... and then he CRUSHED it!!!

Watch the clip ... Harrison takes the bar all the way to his chest -- and gets it up to full lockout without any help (you watching, Odell???)

You could tell Harrison was super impressed with himself ... 'cause he immediately jumped off the bench looking for dap from fellow gym-goers.

"500!! I told you I’d be back for your bi$&h a$$!" James wrote after the lift.

"You just don’t know how many nightmares you gave me - having to relive the failure of not tossing your heavy a$$ off my chest and having to explain to my kids that daddy lost! Lmao JK #500club"

If you're unfamiliar, James is maybe the most impressive NFL weight lifter ever ... routinely tossin' around iron like he did to offensive tackles.

In fact, remember when he was in the gym at 5 AM after a playoff game in 2017?!?!