USA's Justin Gatlin: I'M GOING TO BEAT USAIN BOLT ... Period.

You heard it here first ... Team USA sprinter Justin Gatlin guaranteeing OLYMPIC VICTORY over Usain Bolt at the 2016 Olympics!!!

Gatlin joined the guys on the "TMZ Sports" show on FS1 -- when he told us he's not afraid of the Zika scare in Brazil ... and NOTHING can keep him from competing against his rival at the games.

So, when we asked straight up if he was finally going to crush the Jamaican sensation on the track -- Gatlin laid down the gauntlet ... "I'M GOING TO WIN!"

By the way, Gatlin says he's already got plans for his gold medal when he crosses that line!!!