كتب : Omar AbdelAziz
The former Belgium defender has already announced his Velodrome exit this summer after a dispute with owner Robert Louis-Dreyfus.
Gerets, who is on course to win the Ligue1 with l'OM, set his reasons for choosing the Gulf move.
"Of course, money played a role in it. I cannot dispute that. But this is a real challenge because I know nothing of the place," he told France Football.
"I have a good feeling about this move. I find this very exciting and I love new challenges."
It is believed that Gerets will earn 250,000 Euros as a monthly salary in Riyadh.
Former France captain Didier Deschamps was named as successor to Gerets, who was reportedly linked with Egyptian champions Ahli.