A Roberto Clemente rookie card -- one of the greatest outfielders in MLB history -- is hitting the auction block next month ... and is expected to sell for over $1,000,000!!
TMZ Sports has learned Clemente's 1955 Topps rookie card -- recently graded a PSA 9, indicating "mint" condition -- will go up for auction in September at PWCC Marketplace and could rival the previous record for a Clemente card, $1.107 million.
PWCC officials tell us the '55 Clemente rookie card is in the "upper echelon of quality for vintage trading cards."
The card's front features two images of Clemente in his Pittsburgh Pirates uniform and baseball cap, and the back has stats from when Roberto played in the minor leagues.
"There are just so few copies of this card in this quality in existence that they require the highest level of custodianship possible," said PWCC exec Jesse Craig.
"We're talking about a museum-quality piece here."