The 26-year-old joins for an undisclosed fee having impressed in the Bundesliga last season.
Dejagah also made his international debut for Iran this year.
"I'm very happy to be signing for Fulham and playing in the Premier League," he told the club's official website.
"I have heard good things about the club and once I had spoken to the manager about the project here, I was convinced.
"I am happy to be here and I will give my best for this club and our fans."