TI -- I Just Played My 1st Poker Game ... and Won BIG

TI's got the best beginner's luck on the planet -- because Monday night, the rapper sat down for his first ever live poker game ... and somehow QUINTUPLED his money, winning thousands of dollars.

Sources tell TMZ, TI was fresh off a concert in St. Louis and had some time to kill before his flight -- so he decided to try his luck at the poker table at the Lumiere Casino.

Thing is ... TI had never played a single game of poker in a casino EVER before that night -- and he still cleaned house.

Sources tell us, TI bought in for $500 -- and left the table with $2,500-$3,000.

Tell 'em other broke jokers be quiet.