Boxer Victor Ortiz -- DEMANDS MAYWEATHER REMATCH ... 'I'll Fight Him for Free'

Victor Ortiz is CALLING OUT Floyd Mayweather ... demanding a rematch with the champ and telling TMZ Sports he'll do it FOR FREE!!!!

The 27-year-old former WBC Welterweight champ was out in Malibu this weekend for the 2014 Nautica Malibu Triathlon ... when he issued the challenge.

FYI -- the two fought in 2011 ... with Mayweather emerging victorious after tagging Ortiz in the chin while Victor was trying to apologize for a headbutt.

"He owes me a rematch," Oritz says ... "And it's a free one from me to him."

Ortiz adds, "Mayweather, whenever you say bro ... you're the one that's calling the shots. I can't."

"I'm waiting for you."