NASCAR legend Tony Stewart straight-up socked a heckler in the jaw at a racetrack Friday night ... and the entire incident was caught on camera.
48-year-old Stewart -- a 3-time NASCAR Cup Series champ -- was driving in a sprint car race in Jackson, Minnesota when he claims his engine blew out on him after just one lap.
A ton of fans flocked to the track to see the race star drive that night -- the Jackson Motorplex even advertised it as "Tony Stewart Night" -- so Tony stuck around afterward to sign some autographs.