The midfielder has signed a four year deal with the 'Cottagers', in a deal worth around €35million.
Seri brings with him international experience, having been capped 18 times by the Ivory Coast, and he scored 12 times in 103 Ligue 1 appearances for Nice.
It comes as some coup for Fulham - the player had been linked with a host of high profile clubs, and this time last year he was close to signing for Barcelona.
"I'm delighted to be here, it's a new adventure that starts for me and I'm hoping it will all go well," said Seri.