Boxing superstar Deontay Wilder says there's a damn good reason he rocks that HUMONGOUS gold chain before a fight ... it's so heavy, it beefs up his neck muscles!
... And, yeah, he's DEAD SERIOUS!
"It's so heavy on my neck, it's like training," Wilder tells TMZ Sports ... "It's almost like wearing a weighted vest."
Wilder says the chain is worth more than $100,000 -- and since he's gearing up to fight Luis Ortiz on Saturday night, he decided to break out the big boy jewelry to get his neck right.