The Czech Republic international has made just four appearances for the Blues since joining in 2010 for around £5.2million.
Since then Kalas has been on a series of loan moves and last season helped Middlesbrough gain promotion to the Premier League.
He has once again been sent to the Championship to gain experience, where he will work with former Chelsea midfielder Slavisa Jokanovic at Craven Cottage.
And Kalas admits Fulham’s persistence was a factor in his decision to join them.
“Fulham were really active, trying to reach me, calling me, and I was quite happy to see that interest,” Kalas told Fulham’s official website.
“I could really see that the club and the manager wants me.”