Guantanamo Bay inmate Ridah Bin Saleh al-Yazidi has been repatriated to Tunisia, the US Department of Defense says.
He was found eligible for transfer from the detention facility after a "rigorous interagency review process", a press statement said.
The Pentagon did not say if Mr Yazidi had accepted any guilt.
Since 2002, the Guantanamo Bay detention facility has been used to hold what the US describes as captured unlawful combatants during America's "war on terror".
The camp is part of a US naval base complex in south-eastern Cuba.
According to an unconfirmed report by the New York Times, Mr Yazidi was never charged and was approved for transfer more than a decade ago.
The newspaper also said he had been at Guantanamo Bay since the facility was first set up in 2002.
According to Monday's Pentagon statement, 26 detainees remain at Guantanamo Bay, of whom 14 are eligible for transfer.
Earlier in December, the Pentagon announced that the US had repatriated three other detainees, the Associated Press news agency reported.
The camp was established by the Bush administration to detain the most dangerous suspects for interrogation and the prosecution of war crimes.
Controversy has centred around the period of time detainees have been held without charge and the use of interrogation techniques.