Midfielder Plumain, 18, arrives on a free transfer from the Ligue 2 side.
He comes with a good reputation, having featured for France's U16 side before making his professional debut late last season.
Plumain played 22 times in France's second division, scoring once.
The Whites announced that he will play for their development squad next season, which is managed by former defender Kit Symons.
Fulham's U18 side have won the league in the past two seasons. They also snapped up England Schoolboys midfield Foday Nabay from Birmingham earlier this summer.
Fulham manager Martin Jol has so far strengthened his first-team squad with the signings of goalkeeper Maarten Stekelenburg from Roma, Ghana midfielder Derek Boateng and Athletic Bilbao defender Fernando Amorebieta.