The 18-year-old joins on a deal until June 2016 and goes straight into the Development squad for the 2013/14 Under-21 Premier League season.
A Fulham statement read: "The club is delighted to announce the signing of Ange-Freddy Plumain on a contract until 30th June 2016, with an option to extend for another year."
Plumain scoring one goal in 22 appearances for RC Lens last season and has represented France at Under-16 level.
Fulham manager Martin Jol has admitted he is pursuing 'one or two' new players before the start of the Premier League campaign.
Jol has snapped up Fernando Amorebieta, Derek Boateng and Maarten Stekelenburg this summer and the Cottagers have also been linked with Roma striker Pablo Osvaldo.