It's true -- his name is Marcus, and shocker -- turns out knockout power runs in the family!
We spoke to the Gaethje boys (both on Fight Island getting ready UFC 254: Khabib vs. Gaethje) ... and they revealed how they used to destroy people together, while playing high school football back in Arizona!
Both Gaethje's were 2-way players -- Justin was a QB and strong safety, Marcus was a tight end and middle linebacker.
But, they got their kicks on defense -- where Justin says they were a "brotherly force" ... "We both hit hard!"
Marcus tells TMZ Sports he always knew his bro was a talented athlete but didn't think he'd pursue professional fighting until Justin became a star wrestler in college.
"He met one of those wrestling coaches that was kinda in the fight game and he brought some UFC fighters to the wrestling [gym] and I think that's when he made the decision that he wanted to at least try fighting," Marcus says.
"Other than that, he did a couple backyard fights with gloves on but that was really it."
Justin's career choice has panned out -- he's fighting for the UFC lightweight belt on Saturday.
Justin says he always wanted to be an NFL player -- but he just wasn't big enough.
He's still a huge fan of the Raiders and plans to take his UFC belt to the team after he beats Khabib!
There's a lot more ... Marcus also talks about what it was like growing up in the small town of Safford, AZ -- population 10,000.
"You would never think that this was possible for someone coming from our town [to make it big] ... and then it being my brother? It's kinda cool."
"We've seen, us as a family, have seen all the hard work that he's put in since such a young age and now it's all starting to pay off so it's really cool."