46-year-old Shaker Aamer was held at Guantanamo since 2002 after the US accused him of acting as a recruiter, financier and fighter for Al-Qaeda, as well as being a close associate of Osama Bin Laden, but he has never been charged or put on trial.
He was detained in Afghanistan in 2001 after US authorities alleged that he had led a unit of Taliban fighters and had met Osama.
UK foreign secretary Philip Hammond said Aamer had left the US military base and will return to the UK later today.
The father of four children, Aamer has permission to live indefinitely in the UK because his wife is British.
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He was born in Saudi Arabia in 1968, and lived in the US before settling in Britain, where he married a British woman and, in 1996, became a resident.
Guantanamo was opened by Obama's predecessor after the September 11, 2001, attacks on the World Trade Center in New York.
Andy Worthington, co-director of the 'We Stand With Shaker campaign' said Aamer's lawyer had informed him of the release.
"We hope he won't be detained by the British authorities on his return and gets the psychological and medical care that he needs to be able to resume his life with his family in London," said Worthington.
He was transferred to Guantanamo Bay on February 14, 2002, where he alleged that the maltreatment continued, leading him to become an advocate for prisoners' rights and an organiser of hunger strikes.
He remained on hunger strike as the Obama administration announced last month that he was to be freed.
Rights group Amnesty International, which took up Aamer's cause, called his detention "intolerable".
"We should remember what a terrible travesty of justice this case has been, and that having been held in intolerable circumstances for nearly 14 years Mr Aamer will need to time to readjust to his freedom," Kate Allen, Amnesty International UK director, said.