Abu Ghaith, 48, a Kuwaiti cleric, showed no emotion as he listened to the jury's unanimous verdict by simultaneous Arabic translation through earphones in a Manhattan federal courtroom with a view of the gleaming new World Trade Centre tower.
He will be sentenced in September.
Wearing a dark suit and open shirt, the balding Abu Ghaith cut a very different figure from the charismatic young preacher who appeared in propaganda videos next to bin Laden in a turban and camouflage vest, an AK-47 automatic rifle propped up nearby.
The most damning evidence against him was his own words. In one video recorded on September 12, 2001, he is shown sitting beside bin Laden as al-Qaeda claimed responsibility for the attacks.
In another, filmed about a month later, he warned Americans that the "storm of aeroplanes will not abate". Prosecutors said that this was a direct reference to the "shoe bomb" plot, which was already in the planning stages in Afghanistan, as Badat testified in closed circuit evidence from London.
In a third video, titled "convoy of martyrs", Abu Ghaith was shown preaching over scenes of a plane flying into one of Twin Towers - proof of his role in aiding al-Qaeda by recruiting young Islamic fighters, prosecutors said.
Abu Ghaith recounted his meeting with bin Laden on September 11 2001 when he made an unexpected appearance on the witness stand. The al-Qaeda chief told Abu Ghaith, who would later marry his oldest daughter, that he was "too gloomy" when he predicted American reprisals for the attacks. "We are the ones who did it," he testified that bin Laden told him. "I want to deliver a message to the world... I want you to deliver that message."
The prosecution argued that the videos alone proved Abu Ghaith's guilt.
"This man was not Osama bin Laden's robot," John Cronan, assistant US attorney, told the jury.
"He was not his puppet... He was no accidental terrorist.
"During the most important period of time in al-Qaeda's savage history, Sulaiman Abu Ghaith was [his] principal messenger.... He used his fiery oratory to incite al-Qaeda's growing army of terror in this war with America."
Abu Ghaith's lawyers claimed that his role was religious, even if his rhetoric was hateful. Stanley Cohen, the lead defence attorney, said that he would appeal after the judge rejected a request to call as a witness Khalid Sheikh Mohammed, the self-described September 11 mastermind, who said in a written statement that Abu Ghaith had no military role in al-Qaeda.
He accused prosecutors of seeking to inflame jurors by making constant references to 9/11, even though Abu Ghaith was not charged in those attacks.
