Judge Mentz said Motassadek knew about the attacks
Mounir al-Motassadek, the first man to stand trial over the 11 September attacks, has been jailed for 15 years.
The 28-year-old Moroccan was found guilty in Germany of being an accessory to the murder of more than 3,000 people in the attacks on New York and Washington.
He was also convicted by the Hamburg court of being a member of a terrorist organisation, and of attempted murder and causing grievous bodily harm.
Motassadek was found to have provided back-up to a Hamburg-based terror cell as it plotted to hijack planes and fly them into the World Trade Center and the Pentagon.
The accused belonged to this group since its inception - he knew and approved the key elements of the planned attacks
Presiding Judge Albrecht Mentz
He denied the charges, and defence lawyers have said they will appeal against the conviction.
The panel of five judges handed down its ruling amid tight security outside the court.
Motassadek showed no emotion as the sentence was read out.
"The accused belonged to this group since its inception," said presiding Judge Albrecht Mentz. "He knew and approved the key elements of the planned attacks.
Motassadek is expected to appeal against the verdict
"This group of Arab-Muslim students planned
the attacks out of hatred for the United States and Israel.
"They wanted to strike at the foundations of the United
States with this attack of unprecedented dimensions."
Stephen Push, whose wife Lisa was killed on one of the aircraft used in the attacks and who was present at the trial, welcomed the verdict.
"This is a wise decision," he told Reuters news agency. "I'm only sorry he couldn't get a longer sentence because
someone who is committed to al Qaeda is a very dangerous person
who needs to be kept away from society."
Prosecutors had described Motassadek, an electrical engineering student, as a willing participant in the plot and a willing adherent to the hijackers' ideology.
He was found to have played a substantial logistical role in the preparation of the 11 September attacks.
He managed the bank account of one of the hijackers, Marwan al-Shehhi - which prosecutors have said served as a financing pot for an al-Qaeda cell in Hamburg, and was used to pay for flying lessons in the US.
He also signed the will of the alleged ringleader of the cell, Mohammed Atta, and had attended al-Qaeda training camps in Afghanistan.
But Motassadek's lawyers said they would appeal on the basis that evidence used by the prosecution against him did not stand up.
They said his involvement with Atta and al-Shehhi amounted to nothing more than innocent favours between friends.
Motassadek said that he rejected violence and denied that he had been converted to extremism.