Ruiz Jr, who will be bidding to become Mexico's first heavyweight world champion, replaces Joshua's scheduled opponent Jarrell Miller, who was barred from fighting after failing drugs tests.
"The chills — I'm really excited for this fight," said Californian-born Ruiz, 29, in a statement released by Matchroom Boxing.
He cited Mexican great Julio Cesar Chavez, a world champion in several lighter classes, and former heavyweight world champions Mike Tyson, Evander Holyfield and Lennox Lewis.
"This is my chance to make history, I want to be one of those greats like Chavez, Tyson, Holyfield, Lennox Lewis. I want to be in that category. I'm going to bring the titles back to Mexico. I'm going to be able to change my whole family's lives, my life and all my kids."
Ruiz has won 32 of his 33 professional bouts, with his only defeat coming in a world title challenge against New Zealand's Joseph Parker.
Joshua, 29, who has won all of his 22 fights, will be fighting in the United States for the first time and defending his WBA, IBF, WBO belts.
"Ruiz is a different kind of challenge but one I embrace," said Joshua. "We have worked in the boxing gym week on week and whomever is put in front of me at Madison Square Garden on June 1 will be dispatched in style. The rent will be collected."
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