"I'm thrilled to have taken a job for an outstanding new owner and I'm looking forward to the association," said the British-based Italian jockey.
Last October, Dettori ended an 18-year stint as the retained jockey for the Godolphin stable of Dubai ruler Sheikh Mohammed bin Rashid Al Maktoum.
"I went over to Chantilly and had a nice meeting with Sheikh Joann. We've started an agreement and I'm looking forward to this new venture for me," he told the At The Races television channel.
"I rode a couple of horses for him at Royal Ascot and he was very interested to have my service. I had a long talk with him and he seems a very ambitious owner," added Dettori.
"He's got plenty of horses and he's got plans to expand, so it's a good opportunity for me to get back in the big league."
One of the biggest names in the sport, Dettori ended a six-month ban for doping in May and returned initially as a freelance.