The federal jury on Tuesday rejected Ahmed Omar Abu Ali's claim that Saudi authorities whipped and tortured him to extract a false confession.
Abu Ali, a 24-year-old US citizen born to a Jordanian father and raised in Falls Church, Virginia, could get life sentence on charges that include conspiracy to assassinate the president and providing support to al-Qaida. Sentencing was scheduled for February.
Abu Ali did not testify. Khurrum Wahid, his attorney, said outside the court: "Obviously the jury has spoken but the fight is not over. We intend to use the justice system to prove our client's innocence."
Abu Ali confessed shortly after his arrest in June 2003 at a university in al-Madina, Saudi Arabia, that he had joined al-Qaida and discussed various "terrorist" plots, including a plan for him to assassinate Bush and to establish himself as a leader of an al-Qaida cell in the United States.
But the defence said that he was whipped and tortured into a false confession by the Saudi security force known as the Mabahith.
Wahid suggested that an al-Qaida member arrested by the Saudis falsely fingered Abu Ali to protect other cell members still at large.
Prosecutors say he was never mistreated and confessed voluntarily.
According to prosecutors, Abu Ali went to Saudi Arabia in 2002 with the notion of becoming a "terrorist", and later met al-Qaida's No 2 man in al-Madina.
Stephen Campbell, prosecuting, said: "The true focus of his education quickly became apparent. Instead of studying Islamic law, he began attending secret terrorist training sessions."