The Mexican-American dropped AJ four times to make history as the first heavyweight champion from Mexico and end his opponent’s unbeaten streak.
The fight began with a tense couple of opening rounds as Joshua jabbed to make an impression while Ruiz Jr showed flashes of his impressive hand speed.
In the third, the bout exploded into life as AJ put together a lightning-fast combination and dropped the challenger.
However, as he pursued the finishing blows, he was then caught himself by a gigantic left hook which scrambled his equilibrium.
Two knockdowns resulted from this and the champion only just survived the round by the skin of his teeth.
Watch fight below: