Promoter Eddie Hearn says Andy Ruiz Jr has signed to fight Anthony Joshua in Saudi Arabia despite speculation he has not committed to the bout.
The unified world heavyweight champion, 29, has not shared any fight details on social media since the 7 December rematch was announced on Friday.
His trainer Manny Robles says "there's a reason why" Ruiz has kept silent.
Hearn, who promotes Joshua, said: "Both fighters signed for this fight. The governing bodies have been informed.
Visas, vision and Saudi
At a London news conference where neither Joshua or Ruiz were present, Hearn said the rematch will take place in an open-air stadium in Diriyah, on the outskirts of Riyadh.
Omar Khalil, working for the Saudi agency delivering the event, said fans who purchase a ticket will automatically be issued a visa to visit the country.
Madison Square Garden - where Ruiz stunned Joshua on 1 June - and Cardiff's Principality Stadium were picked as likely venues for the bout but Ruiz made clear he did not wish to fight in the UK, while Hearn played down an immediate return to the USA.
Saudi Arabia has been highlighted as a compromise but the country's human rights record has prompted criticism of the choice of venue for a fight of such magnitude.
"We had approaches from Saudi Arabia, Abu Dhabi, Dubai and Qatar," Hearn added. "We wanted to go somewhere that had a vision for the sport of boxing. We already knew Saudi Arabia was for real and investing in the sport.
"We have to realise that there is another world out there outside of Cardiff and Madison Square Garden. We have an obligation to grow the sport to new areas and regions.
"This event could change boxing forever. If Saudi is going to invest in these fights you could be seeing a big change in the dynamics of the sport, which truly excites me."
Saudi human rights a fight focus
Bouts involving Britons George Groves, Callum Smith, Amir Khan and Hughie Fury have taken place in Saudi Arabia in the past 12 months.
The country has staged big sporting events such as Formula E races and has recently announced it will host the richest event in horse racing history in February.
Such moves are part of a wider strategy to showcase the country as a modern, tourist-friendly destination as it looks to transition away from its oil-dependent economy.
But human rights campaign group Amnesty International has already urged Joshua, 29, to "speak out about Saudi Arabia's abysmal human rights record."
The group said the fight will provide "yet another opportunity for the Saudi authorities to try to 'sportswash' their severely tarnished image".
Khalil, who works for a sports and entertainment agency in Saudi Arabia, said: "We are a demography of 40 million people and 70% are below 24 years old.
"So the appetite for such events is huge. This event is a continuation of the plan Saudi is working on to enhance that populations' quality of life and overall wellbeing."