An extremely rare Giannis Antetokounmpo rookie card just became the most expensive piece of cardboard in basketball ... with the final bid fetching a whopping $1.812 MILLION!!!
As we previously reported ... the signed Giannis card -- listed by Goldin Auctions -- features a piece of a game-worn jersey from the Greek Freak's rookie season.
The card was part of the Panini America's 2013-14 high-end "National Treasures" line ... and features the NBA "Logoman" patch from one of Giannis' rookie jerseys.
The card was graded a MINT 9 by the Beckett Grading Service .