Faez Mohammed Ahmed al Kandari, 38, a Kuwaiti who has been held at Guantanamo for 13 years, was suspected of being a propagandist. He also may have served as "spiritual adviser" to al Qaeda leader Osama bin Laden, according to a US Department of Defense profile.
A US national security review found that the detention of Kandari "does not remain necessary to protect against a continuing significant threat to the security of the United States," the Pentagon said in a statement.
The statement did not say when Kandari would be returned to Kuwait.
The parole-style Periodic Review Board held a hearing in July to consider whether Kandari posed a threat to the United States. In September, it determined his continued detention was no longer necessary.
The board, established by President Barack Obama in 2011, is comprised of six intelligence and national security departments and agencies.
The Pentagon profile said Kandari had complied with officials at Guantanamo Bay since his last review in 2014, which was marked by an outburst from Kandari.
The transfer of Kandari leaves 104 inmates at the facility, which Obama had promised to close. He has not been able to overcome opposition from the Republican-controlled Congress to do so.
