Marshawn Lynch allegedly threatened to "slap the smile" off the face of a 13-year-old boy who was asking for an autograph this weekend ... this according to the boy's uncle.
Neither the boy nor his uncle captured the alleged threat on tape -- but say it all went down in front of a group of witnesses.
Here's the deal ... Lynch (who was in Wisconsin to play the Packers) was leaving the team hotel on Saturday when a group of young autograph seekers hit him up for his signature.
In footage that has already surfaced, you can see Lynch wasn't having it -- and told the group to "back up off me."
But we spoke with Dave Medina -- who was there with his 13-year-old nephew -- and he says Lynch got even nastier when the cameras stopped recording.
"He told my nephew, 'How would you like it if I slapped that f**king phone out of your hand and that smile off your face.'"
Medina says he was absolutely shocked at Lynch's behavior because the group -- comprised mostly of high school kids -- get autographs at the hotel from other NFL players all the time and they've never had an incident before.
We reached out to Marshawn's people for comment -- so far, no word back.