Judge hit the blast in the seventh inning of the Yankees' blowout win over the Blue Jays in Toronto -- officially tying the American League watermark set by Roger Maris back in 1961 -- and one fan was in a perfect position to nab the ball as it crossed over the left field wall.

He had his glove ready ... and when he reached over some railing in front of him to catch it -- the legendary cowhide touched his mitt's webbing!!!

Unfortunately for the guy, it caromed off his hand and landed safely in the Jays' bullpen ... something that clearly crushed the man.

Video from the game's broadcast in Canada showed the guy was absolutely devastated over the fumble -- throwing his mitt and hat in disgust afterward.

Of course, the guy had every right to be sad ... the ball would have been worth a HUGE payday for the Jays fan.

David Kohler of SCP Auctions told TMZ Sports earlier this month it could sell for as much as $250,000 on the auction block!!

As for what ultimately ended up happening to the ball -- Toronto bullpen coach Matt Buschmann initially grabbed it. He gave it to Jays closer Jordan Romano ... who then passed it off to Yankees reliever Zack Britton, who returned it to Judge.

Judge held onto it for a while -- but in a super sweet moment, he turned it over to his mother.