TMZ Sports has learned ... Mays' game-worn and autographed threads from the 1969 All-Star Game -- when Mays played for the San Francisco Giants -- are up for grabs via Heritage Auctions.

The No. 24 gray, orange and black jersey features the SF name on the front ... with a sweet John Hancock from the "Say Hey Kid" himself. It also has "Mays 42/3 1969" and the "W.A. Goodman & Sons" label on the front tail.

The auction house tells us it's taking bids for the jersey -- which was photo-matched to confirm it's the real deal -- on July 26 ... and it's expected to sell for more than $400,000!!!

Of course, it's a super rare piece for any baseball fanatic to have in their collection -- Mays is one of the greatest MLB players of all time ... as he became a 24-time All-Star, a 1954 World Series champion, a 12-time Gold Glove Award winner, and much more over the course of his career.

His legacy has been honored in many ways following his playing days ... including having his jersey number retired by the Giants and New York Mets. The MLB also named the World Series MVP trophy in his honor.