Andy Ruiz Jr. stunned British boxer Anthony Joshua with a seventh-round technical knockout Saturday night to become the first fighter of Mexican descent to claim the heavyweight title.

The 29-year-old native Californian shocked a sellout crowd at Madison Square Garden with his surprise victory, marking one of boxing’s biggest upsets behind Buster Douglas’ win over Mike Tyson for the heavyweight title in 1990.

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"I just feel so good, man," Ruiz said. "This is what I've been dreaming about, this is what I've been working hard for. I can't believe I just made my dreams come true."

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The 270-pound heavyweight knocked down Joshua twice in the third round and did it two more times in the seventh before referee Mike Griffin ended the fight at 1:27 in the round. The 247-pound Joshua was unsteady and seemed to stumble toward the wrong corner after the final blow.

"Sorry I let my fans down, sorry I let my supporters down," Joshua said.

Andy Ruiz, right, punches Anthony Joshua during the seventh round of a heavyweight championship boxing match. (AP)

Joshua (22-1, 22 KOs) and Ruiz were both knocked down in an electrifying third round that had the crowd howling with each heavy hit. The 29-year-old Joshua never seemed to fully recover from the knockdowns.

Ruiz (33-1, 21 KOs) accepted the unexpected title shot against Joshua with just six weeks to prepare after several failed drug tests moved the British champion’s original opponent, Jarrell Miller, to the sidelines.

Andy Ruiz poses for photographs after a heavyweight title boxing match against Anthony Joshua. (AP)

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The underdog went on to make boxing history and win the WBA/IBF/WBO/IBO championships.

"My life is going to change," Ruiz said. "I don't have to show them no more."

The Associated Press contributed to this report.