15 Guantanamo detainees had been sent to the United Arab Emirates in the largest single transfer of the Obama administration, The Pentagon said on Monday.
Now the number of prisoners, detained in Guantanamo bay, decreased to 61, after the transfer of 12 Yemeni nationals and 3 Afghans to Emirates.
The United States had held each of 15 released without trial for about 14 years. Their departure reduced the list of prisoners approved for transfer to 20 men.
In a statement on Monday, the Pentagon said: "The United States is grateful to the government of the United Arab Emirates for its humanitarian gesture and willingness to support ongoing US efforts to close the Guantanamo Bay detention facility."
President Obama wants to close the prison before he leaves office. In February, the White House presented a plan to Congress to close the camp.
Earlier this year on April, nine Yemeni prisoners have already been sent to Saudi Arabia.
The White House also wants to transfer the remaining inmates to the US. But that plan has been blocked by Congress. Also many Republicans remain strongly opposed to bringing inmates to the US, saying they are extremely dangerous and do not belong in civilian prisons.
Ed Royce, the Republican chairman of the House Foreign Affairs Committee, condemned the latest releases, saying: "Once again, hardened terrorists are being released to foreign countries where they will be a threat."
Republican presidential candidate Donald Trump has vowed to keep the Guantanamo Bay facility open, saying he will fill it with "bad dudes"
and "bring back a hell of a lot worse than waterboarding"
The Guantanamo Bay prison, located on an American naval base in south-eastern Cuba, was opened in January 2002 by former US President George W Bush to accommodate foreign terror suspects after the 11 September attacks in 2001.