A ball Patrick Mahomes signed back in high school is hitting the auction block ... and check out the signature -- the NFL superstar had a John Hancock like a pro even as a teenager!!
TMZ Sports has learned the K.C. Chiefs QB put pen to ball back in 2013 ... and now, it's the earliest Mahomes auto EVER to go up for auction.
We're told the story behind the ball is pretty special ... the guys at Lelands say Mahomes and his entire Whitehouse H.S. Wildcats baseball team in Texas signed it for a boy who was battling Leukemia.
Leland's reps say the boy threw out the first pitch at one of the team's games (with Mahomes catching it) -- and later, the kid got the whole team to sign it.
Check out the piece ... in the sweet spot, "2013 Whitehouse Wildcats" is handwritten. Just above it is Mahomes' auto, which features his signature with the "#5."
21 of Mahomes' other high school teammates also signed the ball.
Unclear what it's ultimately expected to fetch -- it is awfully hard to price such a unique item, after all -- but bidding has started at $500.
And, if that price sounds crazy ... consider this -- a ball signed by a teenage Michael Jordan in 1976 just sold this year at auction for $36,000!!!