Joshua was floored four times and beaten in seven stunning rounds in a result to rival any surprise in the sport’s illustrious history.
The previously unbeaten world heavyweight champion lost his IBF, WBA and WBO titles and suffered a major and unforeseen setback in his plan to dominate a division that includes Deontay Wilder and Tyson Fury.
Joshua’s reign began to unravel in a terrible third round.
He looked in full control when he floored Ruiz Jr with a left hook. But, in trying to finish the fight, Joshua was caught cold by Ruiz Jr and sent to the canvas for the first time since his classic fight against Wladimir Klitschko at Wembley.
Joshua stood up but was on wobbly legs and Ruiz Jr pounced, sending him tumbling down again. Joshua clambered back up and needed to be saved by the bell for the end of the round.