British promoter Eddie Hearn announced the highly-anticipated sequel for December 7 a fortnight ago but Ruiz, the WBA, WBO and IBF champion, initially disputed the venue.
Reports suggest Mexican-American Ruiz has been swayed by a larger cut of the revenue than his initial share of $15m for the mega-fight dubbed "Clash on the Dunes".
"I dethroned the king June 1st and made history becoming the first Mexican/American champion of the world," Ruiz wrote in a tweet confirming his acceptance.
"I'm looking forward to ending his career in the desert. I will show the greatness of Mexican boxing in Saudi Arabia."
Ruiz stunned the boxing world by flooring the previously unbeaten Joshua four times and eventually getting a seventh round stoppage at New York's Madison Square Garden in June.
He was hoping for the rematch to be held in New York, Los Angeles, Las Vegas or Mexico City but Hearn held the upper hand in in determining the venue and controversially took the fight to Saudi.
The surprisingly swift confirmation of the fight now raises the prospect of New Zealand heavyweight Joseph Parker featuring on the undercard.
Hearn is eager to produce a super-quality lineup for the Saudi extravaganza and previously suggested Parker's long-awaited clash with rugged British fighter Dereck Chisora would suit that.
The other option for Parker v Chisora is to headline an October 26 fight in London.
Parker has two bouts left on his three-fight deal with Hearn.
Parker has been keeping in shape training in Auckland following his lop-sided TKO of Australian Alex Leapai in the United States in late June.
He is eager for action and ready to locate to Las Vegas to start a camp with trainer Kevin Barry once the fight details are locked in.
A win over Chisora would push Parker back into the title talk.