A rare piece of memorabilia is hitting the auction block -- a concert ad signed by both Marilyn Monroe AND Joe DiMaggio ... and the item could sell for some serious coin!!!
Here's the backstory -- TMZ Sports is told an 11-year-old girl saw the couple having dinner at The Beverly Hills Hotel in 1952 ... and approached the Yankees legend for an autograph.
Shockingly, the girl had no idea who Monroe was ... so Joe asked if she wanted the model's Hancock as well.
Both Joe and Marilyn signed the inside of a folded concert advertisement that was on their dining table ... scribbling the words, "Warmest Regards, Marilyn Monroe" and "Best Wishes, Joe DiMaggio."
As for the ad, it's for Eddie Bergman and his orchestra at the hotel's Palm Terrace (we're assuming the dude had bangers).
Get this -- there are 9 holes in the old flyer, which came after the girl used a tack to pin her prized possession on her corkboard in her room!
Now, she's ready to part ways with the item -- it's hitting the market Dec. 11 ... and there's a chance it sells for a fortune!!!
A massive amount is not unusual for the celebrity couple ... just this summer, an autographed baseball by Monroe and DiMaggio sold for $384k.
DiMaggio was one of the greatest MLB players ever ... some of his accolades during his 13-year career with the Yanks include winning 9 World Series, 3-time AL MVP and 2-time AL batting champion.
The 13-time All-Star married the famous singer/actress in 1954 and divorced 9 months later.