The 26-year-old has been linked with a move to the Premier League for some time, but the Cottagers have managed to fend off interest from Gunners Arsenal, Borussia Dortmund and Napoli among others to sign him.
Seri played 103 times for Nice and 18 global caps with Ivory Coast.
Seri's competitive debut for Fulham could come on the opening day of the Premier League season when Fulham host London rivals Crystal Palace. This is a big day for Fulham.
Seri has penned a four-year contract and revealed he is excited for a new challenge in England's capital.
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"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".
Barcelona legend Xavi Hernandez believes Seri has everything it takes to play for the La Liga giants, and should that be the case, Fulham might have just secured one of their best ever signings of all time in the brilliant Ivorian.
The midfielder was a key part of the Nice midfield last season, making 29 appearances as Nice went on to finish eighth in Ligue 1 last season.
Fulham vice-chairman Tony Khan was overjoyed with the new addition, adding: "I'm delighted to announce that Fulham Football Club has signed midfielder Jean Michael Seri from Nice".
"I'm also happy to have joined up with a good friend, Jean Michael Seri".