It was a high-stakes day at TMZ, when Harvey challenged UFC honcho Dana White to a high-stakes game of Blackjack.

Dana was here to promote Saturday's UFC 324 -- the first fight on Paramount+ -- featuring Gaethje vs. Pimblett.

Dana agreed to play the game that has made him a legend at Vegas casinos ... and it turned into a nail-biting contest of the expert vs. the novice. Harvey's decisions made it clear ... he didn't know what the hell he was doing.

Dana was playing for the Boys & Girls Club of Southern Nevada, and Harvey was playing for the L.A. LGBT Center.

BTW ... Dana was such a baller, he pulled $10K in cash out of his pocket!

Watch the full match on YouTube!