A jury found Ahmed Omar Abu Ali, 24, guilty on nine counts, including conspiracy to kill Bush, conspiracy to hijack a plane and offering to aid Osama bin Laden's terror network, according to Edward Adams, a spokesman for US District Court in Alexandria, Virginia.
"This conviction is the result of extraordinary law enforcement work and international cooperation," US District Attorney Paul McNulty said in a statement.
"It serves as a clear warning to all that terrorists can and will be brought to the bar of justice."
Abu Ali had pleaded not guilty and his defense lawyers said he was tortured into making false statements in Saudi Arabia, where he was arrested in June 2003.
But Judge Gerald Bruce Lee admitted the confessions as evidence over the objections.
Judge Lee set a sentencing hearing for February 17, 2006 and Abu Ali could face life in prison.
The indictment against him said Abu Ali and another conspirator discussed two options for assassinating Mr Bush: "an operation in which Abu Ali would get close enough to the president to shoot him in the street, and an operation in which Abu Ali would detonate a car bomb."
Abu Ali received a religious blessing from another co-conspirator to carry out the assassination plan, the indictment added.
Prosecutors said Abu Ali admitted he discussed other plots with other members of his Al-Qaeda cell, including crashing planes into targets in the United States, killing members of Congress and blowing up US military aircraft.
Torture claims
The defence case in the three day hearing centred on allegations that Abu Ali had been tortured into making a confession in Saudi Arabia.
Born in Texas, Abu Ali's parents were Jordanian. The family lived in Falls Church, Virginia, a Washington suburb, until Abu Ali went to study at the Islamic University of Medina in Saudi Arabia.
He was detained in Medina in a sweep following bomb attacks in Riyadh on May 12, 2003 that left 34 dead.
The Washington Post said that Abu Ali was among 20 men detained on suspicion of belonging to a jihadist cell linked to the Riyadh attacks.
Abu Ali was held in Saudi Arabia until February this year, when he was handed over to US authorities.
At one hearing in February he offered to show the judge the scars from his alleged torture in Saudi Arabia.
Abu Ali's parents took legal action against the US government in 2004 alleging that Washington had pressed the Saudi government to detain their son, even though US officials knew he would be tortured.
In December, US District Court Judge John Bates in Washington ordered the Justice Department to state what role, if any, US officials played in Abu Ali's arrest.
The Justice Department refused, arguing that since he was detained abroad Abu Ali was outside the jurisdiction of US courts.