MLB Star Shane Victorino -- I'm Training in MMA ... to Get JACKED for Baseball

Pitchers are gonna think twice before throwing inside on Shane Victorino next season ... 'cause TMZ Sports has learned, the Red Sox star is on a HARDCORE mixed martial arts training binge.

"The Flyin' Hawaiin" is getting into fighting shape with "UFC Fit" -- same MMA workout routine that helped UFC super stars like Johnny Hendricks and Thiago Alves get JACKED before fights.

Shane tells us, "As an athlete, I’ve taken part in a lot of training programs ... and UFC Fit takes things to a whole new level.”

Don Gold -- one of the honchos at the program -- tells us women's UFC star Ronda Rousey uses the system too ... "Mike Dolce and the UFC Fit team just recently trained Ronda for her UFC title defense and got her in the absolute best shape of her career."

Bottom line ... watch out for Shane when the benches clear next year.