The 22-year-old midfielder joined the London club from Palermo on a four-year deal in the summer of 2011 but failed to make a big impact at Craven Cottage.
The Switzerland international scored three goals in 56 appearances in al competitions for Fulham, who suffered relegation from the Premier League at the end of last season.
Kasami is delighted to have moved to Olympiakos, who won the Super League last season and will feature in the group stage of the Champions League in the 2014/15 campaign.
"I am very proud to sign the contract and be part of such a big club as Olympiakos," the youngster told the Greek club’s official website.
"I am looking forward to play at the International Champions Cup against the top clubs and then to play Champions League football.
"I want to say to our fans that I am proud to be a player of Olympiakos and hope to celebrate many times with them and win many titles.
"I can’t wait to play in our stadium and score my first goal."