Joey Fatone is so dead serious Justin Timberlake is NOT reuniting 'NSYNC during the Super Bowl halftime show, he bet our photog $1,000!!!

And the best part ... the bet was made IN MINNESOTA ... just a few miles from the site of the big game!

JT and Joey have been denying it for months, but rumors are swirling and our photog is clearly convinced Justin's getting the band back together on Sunday.

So, when Joey called our guy out Thursday night -- our guy put his money where his mouth is.

Better hope Lance, Chris and JC hop on a flight quick ...