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 … which had the most hardcore of collectors salivating over the thing.

At the time, Goldin told us it could become the most expensive basketball card in modern-day history … and they were right — with the Giannis card edging out the LeBron James rookie card that sold for $1.8 million in July.

Now that’s an INSANE amount of coin … but it’s still nowhere near the Mike Trout rookie card that fetched $3.93 million earlier this year.