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 .