The 20-year-old joined Lille from Bastia in January but never kicked a ball in anger for Les Dogues, instead heading straight back to Corsica on a six-month loan deal.
He would score 10 league goals for Bastia before adding three more in Turkey as France claimed the Under-20 World Cup trophy in July.
That burgeoning promise inspired Marseille to secure his signature for an undisclosed fee and OM president Vincent Labrune told his club's official website: "We are obviously very pleased to have reached a positive conclusion to our long negotiations with our counterparts at Lille.
"Most importantly, we are delighted that Florian could bring his talent to our team. He has all the qualities needed to win the hearts of our supporters. He has shown his desire to wear the Olympique shirt for many weeks."
The Orleans-born attacking midfielder started his career with Grenoble but moved to Bastia when the south-east outfit were liquidated in 2011.