TMZ Sports talked to 37-year-old Johnson days after securing the top spot on the podium at the Master 2 featherweight brown belt division at the IBJJF Masters Worlds ... where he went 6-0, outlasting the 40-competitor field.

"I just fell in love with it. The art of it. And, using the Gi as a tool to be able to strangle your opponent or be able to manipulate their body in certain ways has been fascinating to me," DJ told us.

As for where winning the gold medal ranks among his career accomplishments ... it's near the top.

"It's up there. Everything I've accomplished in my career, it's up there," Demetrius told us, adding ... "I haven't competed in a jiu-jitsu tournament since I was 18 years old, now I'm 37. To go be able to compete with my son in the same tournament, it's up there."

That's saying something when you've won 12 UFC title fights ... and 3 in ONE Championship.

DJ says he had such a great experience competing that he's without a doubt going to enter future tourneys ... as soon as his body heals up from injuries he sustained training and competing.

As for winning his entire weight/belt class was enough to earn DJ a black belt ... not yet!

"The way my professor views black belts, or belting people is based on potential, I still have so much more to learn, and he was like, 'You could get your black belt but you're not learning the steps you need to prepare yourself for that black belt.'"

"'You just won a tournament, congratulations. But, the tournament is done. You won it. We get back in the gym, we move forward, and continue our steps to better ourselves as the best athlete we can be or best jiu-jitsu players we can be."