The 18-year-old arrives at Craven Cottage on a free transfer and will be part of the Fulham development squad that will compete in the Barclays Under-21 Premier League.
Plumain represented France at Under-16 level and he made 22 appearances for Lens in the French second division last season, scoring one goal.
He has signed a contract until June 30, 2016, with the option of extending the deal for another year.
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.
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