Mohamed Mohamud has been found guilty of trying to use a weapon of mass destruction in Portland.
A jury delivered the verdict at 3 p.m. Thursday, one day after attorneys made their closing arguments.
Mohamud's sentencing is scheduled for May 14.
Mohamud, 21, was arrested after the FBI said he attempted to detonate a fake bomb during Portland's Christmas tree-lighting ceremony in November 2010. The fake bomb was supplied by FBI agents posing as al-Qaida recruiters.
Greg Fowler, special agent in charge of the FBI's Portland division, issued a statement after the verdict was announced saying the jury decided Mohamud's fate, "while balancing the needs for safety and justice."
"His actions showed little regard for the rights and responsibilities that come with being an American or respect for the lives that he was prepared to take," the statement said.
The defense argued that Mohamud was entrapped by the FBI.
"I thought the FBI took it too far and I think we are hopeful those mitigating factors will be considered at sentencing," said defense attorney Stephen Sady.
Osman Barre, Mohamed Mohamud's father, testified that he was concerned for his son's safety when he contacted the FBI in 2009.
Barre says Mohamud told him he was planning to fly to Yemen to learn Arabic, according to the Associated Press.
Barre says contemporary news accounts of Somali-American teenagers joining the mujahedeen in Somalia persuaded him to contact the FBI and say he feared his son was being brainwashed by al-Qaida recruiters.
But Barre said Monday he now thinks it was an elaborate FBI sting that "brainwashed" his son.
Mohamud did not take the witness stand.
Assistant U.S. Attorney Ethan Knight said Wednesday in closing arguments that Mohamud was predisposed to terrorism when he was 15 years old, the age he told undercover agents he began to think about committing violent jihad.
Knight said jurors may have disagreed with the FBI's methods in its undercover sting operation, the Associated Press reported, but they couldn't argue that Mohamud intended to kill thousands at Portland's 2010 Christmas tree-lighting.
"We tried to present all the evidence as fairly and as clearly as we possibly could," Knight said. "And in some sense, the jury clearly saw that and agreed with it within the law and found the defendant guilty."
Mohamud's parents attended much of the trial, but were not in the courtroom to hear the verdict. They could not be reached for comment.
"I think we all kind of expected that [verdict]. We didn't expect to get any justice," said Saba Ahmed, a member of the local Muslim community who knows Mohamud.
She added, "We wish the FBI had tried to stop him instead of encouraging that behavior. When I was at the trial, I saw how the FBI undercover agents showed him training videos produced by the U.S. government on training jihadi errorists. That sort of stuff, Beaverton teenagers don't think about that stuff."
The trial lasted 10 days.
The defense had filed a motion for a mistrial before the verdict was reached, saying a question by the jury was answered in a confusing manner by the court.
Mohamud's attorneys said the response given to jurors needed to be withdrawn and a new statement written up. If that wasn't done, the defense said the alternative was a motion for mistrial.
Judge Garr M. King issued an order denying the motion for a mistrial Thursday afternoon.