Jim Jones -- I Dropped $20k on Champagne ... IN ONE NIGHT!

Rapper Jim Jones dropped $20,000 at a NYC nightclub last weekend -- buying 32 bottles of champagne ... and after that, dude hooked up his server with a PHAT ASS tip ... TMZ has learned.

The "We Fly High" rapper took over Greenhouse on Saturday night to throw a little birthday shindig for a good friend ... and apparently, money was no object.

Jim ordered 32 bottles of Moet Nectar Rose champagne -- totaling around $20k -- and after he was all bubbled out, he stacked on an extra $6,000 as a tip.

We spoke with Jones ... who tells us "We did 32 bottles because it was my man's bday, and where he's from you pop a bottle for every year of your birth, and that's exactly what we did for him."