The 20-year-old moves to the Velodrome on a five-year deal from Lille without having made an appearance for les Dogues
The 20-year-old joined les Dogues from Bastia in January for just €3.5m but after starring in the second half of the season on loan with the Corsicans, attempted to renegotiate his deal with the club.
However, LOSC steadfastly refused to offer him an improved contract and he has trained just once with his team-mates since the showdown earlier this summer.
And Marseille have swooped to bring him to the Stade Velodrome and club president Vincent Labrune is delighted with the switch.
"We are obviously very pleased to have found a positive outcome to these long talks with Lille," he told OM's official website.
"More importantly, we are delighted that Florian could bring our talent to our team. He has all the quality to win the hearts of our fans."
Thauvin helped France to victory in the Under-20 World Cup this summer but is yet to make a full international appearance.