Mike Tyson jumped the gun with Jake Paul ... slapping his boxing opponent in the face at the final face-off before their big fight ... and it's all on video.
Iron Mike and Jake got face to face after Thursday's weigh-in ... with Tyson slapping Jake across the check.
Mike landed a right-hand slap flush on Jake's face and then appeared to throw another punch below the belt that missed ... before being pushed away from Jake.