'78 Triple Crown Trophy -- Got a Million Bucks? ... Rare Cartier Prize For Sale

One of the RAREST prizes in all of pro sports is about to be up for grabs -- the 1978 Triple Crown trophy ... and it could be yours if you've got a MILLION DOLLARS to spend.

The silver trophy -- which has been made by Cartier since 1950 -- was awarded to "Affirmed" in 1978 ... the last horse to win the prize before "American Pharoah" clinched it earlier this month.

We spoke with famed sports collector Timothy Robins who tells TMZ Sports he bought the trophy from the family of Affirmed's trainer last year -- but now, Tim has decided it's time to part ways with the legendary piece.

Robins has put the Trophy up for sale on Championship-Rings.net -- he's asking for a cool $1 MILLION!

Good news for the new owner -- we're told the trophy is currently in Switzerland where it's getting a thorough cleaning before it moves on to the next phase of its life.