Jake Gyllenhaal -- DUDE CAN FIGHT ... Says World Champion Co-Star

Jake Gyllenhaal is no boxing fraud -- the guy can brawl for real ... this according to a real deal ex-kickboxing champ who worked with Jake on his new boxing flick.

TMZ Sports spoke with David Buglione -- a former U.S.K.B.A World Welterweight Kickboxing Champ -- who worked closely with Jake as his stunt double in "Southpaw."

Long story short ... Buglione says Jake trained so hard for the role, he feels he made himself into a legit boxer.

"Without a doubt he could compete. The time he put into the role, the way he transformed his body. It was incredible."

"He started training 3-4 months before the thing even started. By the time we got into it he'd picked up angles, knew how to move and pivot, and sit on his punches."

Lesson? Don't judge a book by it's cover ... or Bubble Boy might just kick your ass.