UFC's Jon Jones -- 'LET'S FIGHT ALREADY' ... Says NFL Star Bro

UFC champ Jon Jones is being CALLED OUT by his own brother -- NFL star Chandler Jones -- who tells TMZ Sports he's ready and willing to step in the octagon with JJ 'cause he's 100% confident he could beat his ass.

"Me and my brother used to fight all the time when we were young ... and I'm pretty sure I can take him down now. I am way bigger than he is," Chandler tells us.

For the record, Jon is the reigning UFC light heavyweight champ. Chandler just won the Super Bowl.

By the way, Chandler is dead serious ... and says he wants to set up a fight for charity.

"It will be fun if we actually did a one-round match for charity ... that would be fun. If I win, he would have to donate to the charity of my choice and vice versa."

So, how would one fight such an accomplished mixed martial artist? "I would just throw him to the ground."