The Pentagon says the transfer of 12 Yemeni nationals and three Afghans brings the total number of prisoners down to 61 at the US facility in Cuba.
The released inmates had been held without charge, some for over 14 years.
President Obama wants to close the prison before he leaves office.
The jail was opened by former US President George W Bush in January 2002 to accommodate foreign terror suspects after the 11 September attacks in 2001 and the subsequent US-led invasion of Afghanistan.
Some 779 men have been brought there since it opened.
Afghans, Saudis and Yemenis have made up more than 60% of the prisoners, although about 50 different nationalities have been represented.
The vast majority of the 779 inmates were released under the previous administration of George W Bush. Nine have died at the prison.
Mr Obama has approved regular releases, totalling 161, with detainees heading for resettlement around the world.
Only 61 prisoners remain, of whom 20 are approved for release. The key figures left (all of whom have been charged and are currently in pre-trial hearings) are:
Khalid Sheikh Mohammed – regarded as a senior operative in al-Qaeda. Has been charged over the 9/11 attacks. Reportedly admitted personally decapitating kidnapped US journalist Daniel Pearl in 2002
Ramzi Binalshibh – alleged to be another senior al-Qaeda figure linked to 9/11 – tried to become one of the plane hijackers but failed to get a US visa
Mustafa Ahmad al-Hawsawi – suspected financial chief used by Khalid Sheikh Mohammed to arrange the funding for 9/11
Walid Bin Attash – allegedly helped in the preparation of the 1998 East Africa Embassy bombings and the USS Cole bombing and acted as a bodyguard to al-Qaeda leader Osama Bin Laden
What has happened to those released?
Figures from the office of the director of national intelligence show that, of the inmates released under George W Bush, 21% were confirmed to have re-engaged in militant activity, while 14% were suspected of having done so.
Of the releases under President Obama, only 5% were confirmed to have re-engaged in militant activity and 8% were suspected of having done so.
Many of those released have simply returned to their homelands. Many returning Afghans harbour resentment both at why they were sent to Guantanamo Bay and their treatment there. Many still report harassment by security forces although they insist they pose no threat.
Among the countries taking inmates in releases under Mr Obama are the UAE, Georgia, Senegal, Saudi Arabia, Slovakia, Morocco, the UK, Mauritania, Oman, Bosnia, Montenegro, Algeria, Kazakhstan, Uruguay, Estonia and El Salvador.
Inevitably the results have been mixed, given cultural differences.
Uruguay took six Arab men in December 2014 but later said it would take no more, arguing the ex-inmates had struggled to adapt to the country.
One inmate, Abu Bakker Qassi, a Uighur from northwestern China, found himself working as a pizza chef cooking halal Italian food in Albania.