The likes of Chelsea and Arsenal were credited with an interest in the Ivory Coast international, but Fulham have beaten their London rivals to his signature in a major coup for the newly-promoted Cottagers.
Seri has now put pen to paper on a deal worth a reported £18m which will keep him at Craven Cottage until June 2022, with the option of a further 12 months on top of that.
"It was a long day yesterday so it was great to get the move finalised," Seri told the club's official website.
"Now, I'm ready to get started and looking forward to meeting the squad and getting to know them, the head coach and his staff better. I'm delighted to be here, it's a new adventure that starts for me and I'm hoping it will all go well."
Owner Tony Khan added: "I'm delighted to announce that Fulham Football Club has signed midfielder Jean-Michael Seri from Nice. 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.
"So, this is a big day for everyone associated with Fulham and working at Motspur Park, and I trust all Fulham supporters are collectively as thrilled and ecstatic as we are that Jean-Michael will be joining our squad as we return home to the Premier League!"
Seri's competitive debut for Fulham could come on the opening day of the Premier League season when Fulham host London rivals Crystal Palace.