The Syrian-born Spanish national, Imad Eddin Barakat Yarkas, 43, known as Abu Dahdah, was jailed by Spain's high court along with 17 men convicted on charges of aiding al-Qa'ida. Two others were cleared of helping to plot the attacks, though one, Driss Chebli, was sentenced to six years for collaborating with a terrorist organisation.
The 17, mostly of Syrian or Moroccan origin, were sentenced to between six and 11 years' jail for offences including belonging to or helping a terrorist group, possession of weapons, forging documents and fraud.
Yesterday's verdict represents the most significant conviction in the world of those implicated in the 2001 suicide attacks that killed 2,973 people.
Yarkas's sentence falls short of the 75,337 years for murder sought by the prosecution. It reflects the enormous difficulties in establishing proof in a case where none of the accused was ever near the scene of the crime. Prosecutors wanted a sentence based on 25 years' jail for the murder of each of the victims.
The trial of Yarkas and the other suspects jailed since 2001 was from April to July.
A journalist for the Arabic television network al-Jazeera, Tayseer Alouny, who interviewed Osama bin Laden in Afghanistan in August 1997, was jailed for seven years for collaborating with al-Qa'ida.
Defence lawyers argued the case consisted of doubts and suspicions rather than concrete evidence. The defendants were among a group of 41 suspects indicted by Spain's campaigning judge, Baltasar Garzon.
Judge Garzon has said Spain was a base for hiding, recruiting and financing al-Qa'ida members before the attacks on the US.
Yarkas was accused of preparing a meeting in Tarragona on 16 June 2001 with al-Qa'ida operatives including Mohamed Atta - the suicide hijacker who three months later crashed a plane into New York's twin towers - to decide last-minute plans. Yarkas's cell was dismantled in November 2001. Police believe many of its members took part in the Madrid train bombings last year.