Yemeni national Adnan Farhan Abdul Latif was found dead in his maximum-security cell at the U.S. naval base in Guantanamo Bay, Cuba.
Amnesty International said the death was a "tragic reminder of the numbing cruelty of the U.S.A.'s indefinite detention regime" at the facility. The organization added there was "urgent need to resolve the detentions."
Latif's lawyer, David Remes, said the Yemeni was arrested in Afghanistan in 2001 on charges of belonging to the Taliban. He was transferred to Guantanamo Bay in 2002.
Remes said Latif wasn't charged with a crime, was cleared for transfer but remained imprisoned after the U.S. Supreme Court declined a challenge in June.
The Center for Constitutional Rights dubbed Latif "the face of indefinite detention."
Amnesty International said Latif was the ninth detainee to die at Guantanamo Bay. Six of the eight previous deaths were suicides.
It was unclear what led to Latif's death though various media reports state he had a history of hunger strikes.
There are an estimated 167 inmates at Guantanamo Bay, down from the May 2003 high of 680.