Seri, 26, was close to completing a move to FC Barcelona last year before the move collapsed with Nice reluctant to let their star midfielder leave after qualifying for the UEFA Champions League.
Arsenal, Tottenham and Paris Saint-Germain were all reportedly keen on the diminutive playmaker, who's style of play often brought comparisons to Blaugrana legend Xavi Hernandez.
"Jean Michael arrives with Premier League qualities as well as great promise for the future, which is why he has been at the very top of our target list for a long time," Fulham owner Tony Khan told the club's website.
The Cottagers have also completed a deal for Seri's former Nice team-mate Maxime le Marchand.