Turns out Johnny Manziel really WAS raking in 5 figures signing autographs after his Heisman season ... 'cause the ex-college superstar is admitting to making at least $33k off his signature before suiting up for Texas A&M in 2013.

Remember, the NCAA launched a massive investigation into claims Manziel was making dough signing memorabilia (a no-no for college athletes) while in Miami for the 2013 BCS National Championship game ... but found zero evidence of the QB getting paid.

Johnny Football is now coming clean about the whole ordeal ... breaking down the entire thing with Barstool Sports' "Bussin' With The Boys" podcast.

"This guy comes up behind me, he's like, 'Yo, how would you wanna make 3 grand?'" Johnny said. "I turn around, I'm like, f*** yeah, bro!'"

Manziel said he had about $65 in his bank account at the time and wanted some extra cash ... so he followed the guy to his condo to sign "probably 10,000 pieces."