A nephew of Ali bin Kahlah al-Marri said he was in good health and had been welcomed at Hamad International Airport by family and friends.
Marri, who is a US resident, had been serving a 15-year sentence after pleading guilty in 2009 to one count of conspiracy to provide material support to a terrorist organisation.
Prior to that he had been held by the authorities for eight years, initially on credit card fraud charges, but then in solitary confinement for six years after being declared an "enemy combatant" and put in military custody.
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Enemy combatant is the term used by the George W Bush administration to refer to alleged members of Al-Qaeda.
Details on where Marri was held and the terms under which he was released were not immediately available.
His nephew, Saleh Kahla, said Marri had received a joyous welcome.
"Thank God his health and spirits are very excellent. His spirits were better than ours as we did not expect him to be in such good state."
Marri, who is married and has five children, originally went to the United States to study.