Haroon Aswat, 40, who suffers from paranoid schizophrenia, was extradited from Britain to the United States in October.
That was in spite of an earlier 2013 ruling by the European Court of Human Rights in Strasbourg that said the move could breach articles prohibiting inhuman and degrading treatment.
But the court reversed its decision today, saying it was satisfied Aswat was receiving "appropriate treatment" in the US.
Aswat is accused of setting up a training camp in Bly, Oregon in the United States between June 2000 and December 2001 for people wanting to go to fight in Afghanistan.
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Aswat pleaded not guilty to four counts of conspiracy and providing material support to Al-Qaeda in a New York court in October.
His trial is due to begin in June and he faces up to 35 years in prison if convicted.
Aswat was arrested in 2005 in Zambia, extradited to Britain and moved to a psychiatric hospital in 2008 until being sent to the US.
After travelling to Bly, US officials say he spent two months in Seattle living at a mosque where he witnessed men being given additional coaching on how to handle weapons.