Television images from Biggin Hill Airport south of London showed the arrival of a small white aircraft. A government spokesman said Mr Aamer was now back in Britain.
Britain said last month that Mr Aamer would be released,and Foreign Secretary Philip Hammond said earlier on Friday that the former detainee was en route to Britain,a move welcomed by supporters.
"Today is a day for welcoming him back and hoping that he is healthy and well and that can join his family at long last,"Kate Allen,British director of Amnesty International,told Sky News.
In Washington,the Pentagon said it reviewed Mr Aamer's case and concluded he should be released.
"As a result of that review,which examined a number of factors,including security issues,Mr Aamer was unanimously approved for transfer by the six departments and agencies comprising the taskforce,"a Pentagon statement said.
Mr Aamer was cleared for release by US authorities in 2007 but not freed until now. During a debate in the British Parliament this year,some British legislators expressed concern that Mr Aamer was still in Guantanamo because he"had seen too much"and could be still pose a threat to the United States if released.