The 18-year-old Frenchman has joined the Cottagers from the Ligue 2 side and will be a part of the club's development squad for next season's Under-21 Premier League
The 18-year-old arrived on a free transfer from the Ligue 2 side and has signed a contract until 2016 with the option of an additional one-year extension.
He will join up with the Cottagers' development squad for next season's Under-21 Premier League campaign, under the management of former Fulham defender Kit Symons.
Plumain made 22 appearances for Lens in the second tier of French football scoring one goal and has also represented the French national side at under-16 level.
Manager Martin Jol says he still wants "one or two" more signings this summer following the arrival of new owner Shahid Khan at Craven Cottage.