A game-used Sandy Koufax Dodgers jersey -- worn during his insane 1963 season -- just sold at auction for nearly $430k ... TMZ Sports has learned.
Koufax was pretty much unstoppable in '63 ... racking up 25 wins on his way to becoming the National League MVP and becoming the first person to win the Cy Young Award by a unanimous vote.
Koufax also won the World Series MVP that year when the Dodgers beat the Yankees.
As for the jersey, it's unclear which specific game it was used in ... but authenticators were able to confirm Koufax used it at one point during the season.
The jersey -- a road jersey -- was also signed by Koufax who wrote, "1963 W.S. Champs."
Bidding started at $150,000 on the Goldin Auctions website -- and eventually sold for $429,625. The identity of the winning bidder has not been revealed.