Boxing Legend Joe Frazier's Jock Strap Hits Auction Block ... Worn During Ali Fight

The jock strap that protected the manhood of boxing legend Joe Frazier 45 years ago when he beat the GOAT, Muhammad Ali, is about to hit the auction block ... TMZ Sports has learned.

The jock dates back to Ali-Frazier I in 1971 ... known as, "The Fight of the Century," one of America's most historic sporting events. Frazier -- who, like Ali, was undefeated -- won by unanimous decision.

Frazier gave the jock to his buddy the night of the fight ... who held onto it for almost 40 years before selling it to a memorabilia dealer in 2009.

The collector hooked up with Goldin Auctions ... who will now be auctioning off the super rare item starting July 11th. Starting bid is $5K.

FYI -- a Nolan Ryan jock strap worn by the Hall of Famer during his 7th no-hitter reportedly once sold for $25k.

Best part about this jock? It's NEVER BEEN WASHED!!

Happy bidding!