One of the most rare LeBron James cards EVER has just been put up for sale -- a 1-of-1 autographed rookie card -- and it can be yours if you got an extra $1.2 MILLION lying around!!!

TMZ Sports has learned ... the collectible SP Authentic Exquisite Endorsements autographed card is from King James' 2003-2004 season with the Cleveland Cavaliers.

There literally isn't any other card like it in existence ... plus, it's in mint condition with a 9 PSA grade -- after all, it was packed and saved for nearly 2 decades!!

Gotta Love Rock and Roll Auctions tells us ... "This may very well be one of his finest rookie cards ever printed, and it is the first time it has surfaced in nearly 20 years."

The bidding starts August 11 ... and is expected to sell anywhere from $700k to $1.2 million!!!!

That's some serious coin ... but LBJ's rookie cards have been bankin' in big money for the past year!

In April, his rookie patch autographed card sold for $1.72 million and last June, a 9.5 graded card sold for $1.8 mil.