Fener have coughed up €4.5 million for Topal, who has signed a four-year contract.
"I'm so excited and happy. There are no words to describe how big Fenerbahce are. I'm proud to be a part of this club," he told the club's official television channel.
"I want to show my gratitude to Fenerbahce fans. They haven't shown me any animosity. I will do everything to deserve this shirt. I will fight to be accepted by the fans."