The 26-year-old, who has 18 international caps and scored 12 goals in 103 appearances for the Ligue 1 side.
The length of his contract has not been announced, while Fulham have the option to extend Seri’s deal, which runs to 2022, by a further year.
Seri said: “I’m delighted to be here, it’s a new adventure that starts for me and I’m hoping it will all go well.
“It was great to get the move finalised.”
Seri moved to Europe from his homeland in 2012 when he joined Porto on loan, before switching to fellow Primeira Liga side Pacos de Ferreira the following year.
He signed for Nice in 2015 and has topped the Ligue 1 rankings for passes played and completed for the past two seasons.
Fulham co-owner Tony Khan said: “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 Football Club (@FulhamFC) July 12, 2018