In November, Germany's highest appeals court in Karlsruhe found the 32-year-old guilty of abetting the murder of 246 passengers and crew who died on four aircraft used in the attacks of September 11, 2001.
The decision overturned a ruling in 2005 which had convicted Motassadeq of belonging to a "terrorist organisation" and handed him a seven-year jail sentence, but cleared him of abetting mass murder.
That court said he was a lower-tier member of the group led by Mohammed Atta, the leader of the suicide attacks, and Motassadeq's lawyers say he knew nothing about the plot to fly aeroplanes into targets in New York and Washington.
But prosecutors successfully argued in November that under a "division of labour" within Atta's group, Motassadeq played a key role in running the financial affairs of other cell members and covering up their absences from Germany before the attacks.
"I swear before God that I did not know that they were in America. I swear before God that I did not know what they were planning"
- Mounir El Motassadeq
The court heard arguments on Monday from both sides and rejected a petition from Motassadeq's lawyers to suspend the hearing. His lawyers had argued it was unconstitutional because it had been set up purely to determine Motassadeq's sentence.
Motassadeq's case has strained Berlin's relations with Washington as German courts tested how far the United States would go in giving sensitive evidence.
German authorities have indicated that they will deport him to Morocco once he has served his sentence.
He was arrested shortly after the suicide attacks on the World Trade Centre and the Pentagon and has spent the past five years in and out of German courts.
Motassadeq can appeal against the 15-year sentence, but not against his conviction.
Zacarias Moussaoui, a French citizen of Moroccan descent who received a life sentence from a US court last May, is the only other person convicted of having any ties to the attacks.