There have been serious talks of a rematch ever since Ruiz shocked the world and took AJ's heavyweight title back in June ... and now Eddie Hearn's Matchroom Boxing promotion has confirmed the fight is set for December 7.

The timing of the fight comes as no surprise ... but the location seems to come outta left field.

Of course, we talked to Ruiz last month ... and he said he wasn't gonna fight AJ on his home turf because, "I'm the champion ... We're the A-Side."

The fight -- dubbed the Clash on the Dunes -- will take place near the outskirts of the nation's capital in Diriyah.

Of course, Ruiz was a late replacement who stepped in for Jarrell Miller when "Big Baby" failed a drug test -- with Andy only getting around 6 weeks of training before the June 1 fight.

FWIW ... we spoke with legend Evander Holyfield -- who sparred with a 19-year-old Ruiz back in 2011 -- and he says he wouldn't be surprised if Ruiz won again.