Simon Cowell -- LOSES $20,000 'X Factor' Bet to L.A. Reid

Simon Cowell has TWENTY THOUSAND reasons to cry over Thursday night's "X Factor" results show -- and it's all thanks to a rich-person bet he made with L.A. Reid.

TMZ has learned ... Simon and L.A. made a wager at the beginning of the season ... and if either judge had a contestant win the whole shebang ... the losing judge would cough up $20k.

So when L.A.'s contestant Tate Stevens took home the crown ... he grew just a tiny bit richer than he already is. And since Simon's no sore loser, we're told he plans on cutting the check ASAP.