The former UFC fighter reportedly passed away this weekend after battling an undisclosed disease he's apparently been dealing with for quite a while. Unclear what it might be, though ... the rest of the details surrounding his death have yet to be released.

UFC's Michael Chiesa sent out a post for condolences, writing ... "RIP Anthony “Rumble” Johnson. One of the most prolific KO artists our sport has ever seen. My prayers go out to his family and everyone affected by this tragedy."

Other MMA fighters and journalists also posted tons of tributes for the guy ... the news seems to have shocked just about everyone, as Rumble's been out of the spotlight for a while.

One notable remembrance came from Daniel Cormier, who faced Rumble in the Octagon a couple different times. He writes, "Rest Easy my brother. For a guy who struck fear in so many peoples heart Anthony Johnson was a caring person. From random text to check ins during loss. What a person he was, Rumble will be missed. Sometimes life doesn’t seem fair. Horrible news."

What DC's referring to is the fact that Rumble was notorious for his sheer strength and force -- which translated to brutal knockouts that he'd often put on display in his prime. One good punch could put a guy to sleep, and he certainly had a rep for trying to prove that in bouts.

His record was incredible in its day ... 23 wins, 17 of which were by KO or TKO. He only had 6 losses. Rumble competed in several UFC events over the years -- dating back to 2006 -- but he never won a title. For a while, he was ranked #1 in his weight class, pre-retirement. He parted ways with the UFC intermittently, but officially cut ties in 2020 to join Bellator MMA.

Rumble fought at different levels ... middleweight, welterweight, light heavyweight and eventually heavy in his later years. He had the most success in the latter classes.

He was 38.