The Swiss midfielder arrived at Craven Cottage back in 2011, but following their relegation to the Championship, he has opted for a switch to Greek outfit Olympiacos.
In a message to Fulham fans on Instagram, the 22-year-old posted: "I would like to thank everyone at Fulham football club for their great support, especially to all the fans who have been incredible over the last 3 years I've been here. I wish you all the success in the future."
Meanwhile, on Olympaicos's official Twitter page, they added a picture of Kasami signing his contract:
Kasami made 35 appearances in all competitions for the West Londoners last term.