US citizen Jose Padilla has been found guilty of plotting to kill people overseas and supporting terrorism.
Padilla and his two co-defendants could face life in prison
His two co-defendants, Lebanese-born Palestinian Adham Amin Hassoun and Jordanian-born Kifah Wael Jayyousi, were convicted on the same counts.
All three denied charges of conspiring to murder in Chechnya, Afghanistan, Bosnia and elsewhere from 1993 to 2001. They now face possible life in jail.
Padilla was once suspected of, but not charged with, plotting a "dirty bomb".
The three men are due to be sentenced on 5 December, the judge in Miami said.
The jurors took only a day-and-a-half to find them guilty on all three counts against them.
They were convicted of conspiracy to murder, kidnap and maim; conspiracy to provide material support for terrorism; and providing material support for terrorism.
Padilla's mother, Estela Lebron, said she expected her son's lawyers to appeal against his conviction.
Born to Puerto Rican parents in New York, moved to Chicago when 4
Involved in gang crime as a youth, jailed in 1991
Brought up a Catholic, thought to have converted to Islam in jail
Alleged to have learned bomb-making with al-Qaeda in Pakistan
Meanwhile, the White House was quick to welcome the verdict.
"We commend the jury for its quick work in this trial and thank it for upholding a core American principle of impartial justice for all," said White House spokesman Gordon Johndroe.
"Jose Padilla received a fair trial and a just verdict."
The chief evidence presented against Padilla was what the prosecution called an al-Qaeda application form bearing his fingerprints and date of birth that was found in Afghanistan.
The defence had suggested that Padilla handled the document once he was in custody.
FBI recordings of telephone conversations in Arabic between the defendants were also produced, containing, according to the prosecution, coded references to terrorist activity.
Defence lawyers argued that the three men had links with countries such as Afghanistan and Bosnia because they were involved in humanitarian aid for Muslims and said the charges against them were exaggerated.
A lawyer for Hassoun said he was "very disappointed" by the verdict but that his legal team would continue to fight for justice.
Padilla, a former Chicago gang member of Puerto Rican descent, is a convert to Islam.
He was arrested at Chicago's O'Hare Airport in May 2002 after returning from Pakistan.
He was accused of planning to detonate a radioactive dirty bomb and held in US military custody for more than three years.
Following a long battle between the Bush administration and civil liberties groups, Padilla was transferred to the civilian courts in 2006.
The indictment against him made no mention of the alleged bomb plot.
Padilla has said he was tortured while in military detention, an accusation US officials deny.