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UFC legend BJ Penn was involved in another bar fight in Hawaii -- this time, the 40-year-old was sprawled out on top of another man raining down punches, and TMZ Sports has the video.

Penn was at the Lava Shack on the Big Island on Tuesday when he got into it with another dude -- and in classic Penn fashion, the fight ended up on the ground. We have a pic of Penn at the bar earlier in the night. You can see the clothing is the same.

Penn had his opponent mounted -- and his legs were hooked in -- and he threw at least 4 ferocious shots to the other man's head ... and it looks like all of the shots landed pretty clean.

You can see several men in red "Security" shirts move in and try to break the whole thing up.

Unclear what started the fight -- but we're told law enforcement responded to the scene. It does NOT appear Penn was arrested and cops say they have no record so far.

The situation is similar to a fight Penn got into back in June on Oahu -- where he pummeled a man outside of a strip club while a group of people looked on.

Play video content JUNE 2019 TMZSports.com

Cops were called to that incident as well -- but again, Penn was NOT arrested or charged with a crime.

We reached out to Penn's camp for comment -- so far, no word back.

Penn is slated to fight Nik Lentz in the near future but no date has been set. Dana White has said the fight will be the last of Penn's legendary career.

Penn is a UFC Hall of Famer with victories over legends like Matt Hughes and Matt Serra.