Mounir el Motassadeq was sentenced in January 2007 by a German court to 15 years in jail for his role in the death of 246 passengers and crew aboard hijacked aircraft used in the September 11th attacks.
He was the first person ever convicted for complicity in the attacks on the World Trade Center and the Pentagon.
The Moroccan was flown at midday Monday by helicopter from the the Fuhlsbüttel prison in Hamburg where he has been serving his prison term to Frankfurt, according to Spiegel Online and Bild daily.
Photographs carried by national news agency DPA showed Motassadeq, dressed in a checked shirt and beige trousers, being led blindfolded and handcuffed by two police officers to a waiting chopper.
He has admitted to having links to the hijackers, but he maintained his innocence in a five-year court battle.
The Moroccan was friends with members of a Hamburg-based cell including their leader Mohammed Atta, and helped to cover up their whereabouts while they were taking flying lessons in the United States and handled a bank transfer for them.