Presiding Judge Ernst-Rainer Schudt, announcing the conviction of Mounir el Motassadeq after a yearlong retrial, did not immediately explain the Hamburg state court's reasons for the decision.
El Motassadeq was acquitted of more than 3000 counts of accessory to murder in the 2001 attacks on the United States.
The 31-year-old Moroccan, who in 2003 became the first person anywhere to be convicted in the attacks, looked on calmly as Schudt announced the verdict.
Prosecutors had demanded the maximum sentence of 15 years in prison for el Motassadeq, who was accused of helping pay tuition and other bills for cell members to allow them to live as students while they plotted the attacks.
But defence lawyers sought acquittal for the Moroccan, who acknowledges he was close to the hijackers but insists he knew nothing of their plans. They have criticised the lack of direct testimony from witnesses, including Ramzi Binalshibh, a key 11 September 2001 suspect held by the United States.
Defence lawyers sought direct
testimony from Binalshibh
El Motassadeq was convicted in 2003 on both charges and given the maximum sentence. But a federal appeals court last year overturned the conviction, ruling that he was unfairly denied testimony from al-Qaida suspects in US custody.
For the retrial, which opened last August, the US Justice Department provided summaries from the interrogation of Binalshibh and other suspects, but it has not made the full reports available to the court or allowed the captives to appear in person.
According to the statements, Binalshibh - believed to have been the Hamburg cell's liaison with al-Qaida - said that he and the three pilots alone comprised the cell. Prosecutors have argued that he lied in order to protect the defendant.
"Neither the accused nor Binalshibh have any credibility," prosecutor Matthias Krauss said in closing statements last week. "Binalshibh was attempting to protect his brothers."
Defence lawyer Ladislav Anisic has argued that the statements were in any case unreliable and that the prosecution's evidence was flimsy.
El Motassadeq was released from prison soon after his conviction was overturned last year, and has remained free during his retrial.