Think you got better legs than Willem Dafoe? You might get your chance to prove it ... 'cause the Marilyn Monroe dress he donned for his Super Bowl 50 commercial is hitting the auction block.

You remember the clip ... Dafoe rocked a replica white dress from Marilyn's legendary subway grate scene from "The Seven Year Itch" for a Snickers commercial. Funny stuff.

Now, the folks at Julien's Auctions got their hands on it -- and are putting it up for grabs at their Monroe-themed auction in September.

So, how much is it going to go for?

Hopefully A LOT -- because all proceeds will benefit Feeding America, the nation's largest hunger relief organization.

FYI, Marilyn's original white dress went for over $4.6 million at auction back in 2011 when Debbie Reynolds -- another Hollywood legend -- sold it from her private collection.

That's a WHOLE LOTTA candy bars.