"Finally he is returning home," US Secretary of State John Kerry said, confirming the release of Peter Theo Curtis, a 45-year-old author and freelance journalist whose disappearance had not been previously reported.
Curtis was handed over to United Nations peacekeepers in the village of Al Rafid, Quneitra, in the Golan Heights and, after undergoing a medical checkup, was transferred to US representatives, the UN said.
News of his release came less than a week after a grisly video surfaced showing the beheading of American reporter James Foley at the hands of an Islamic State militant.
"Particularly after a week marked by unspeakable tragedy, we are all relieved and grateful knowing that Theo Curtis is coming home after so much time held in the clutches of Jabhat Al-Nusrah," Kerry said, referring to the Al-Nusra Front, another Islamic rebel group operating in Syria.
Kerry said the United States had reached out to more than two dozen countries for help in securing Curtis' release, and that of any other American held hostage in Syria.
US National Security Adviser Susan Rice said she expected Curtis to be reunited with his loved ones shortly.
Curtis' family thanked both the governments of the United States and Qatar, as well as others who helped negotiate his release.
"My heart is full at the extraordinary, dedicated, incredible people, too many to name individually, who have become my friends and have tirelessly helped us over these many months," his mother Nancy said.
"Please know that we will be eternally grateful," she added, pleading for privacy.
According to the family's statement, Curtis was captured shortly before he crossed into Syria in October 2012 and was held since then "by the militant group Jabhat al-Nusra or by splinter groups allied with Jabhat al-Nusra."
The Islamic State and Al-Nusra, who both have thousands of fighters in their ranks, are rooted in Al-Qaeda in Iraq but the two groups have been openly at war with each other in Syria since early this year.
Details of Curtis' release remain unclear. However, his mother said the family was "repeatedly told by representatives of the Qatari government that they were mediating for Theo's release on a humanitarian basis without the payment of money."
Describing him as a published author and freelance journalist from Boston and Vermont, the family statement said Curtis writes under the name Theo Padnos.
Curtis' mother recalled Foley's fate, saying she got to know the late journalist's family.
"We are so relieved that Theo is healthy and safe and that he is finally headed home after his ordeal, but we are also deeply saddened by the terrible, unjustified killing last week of his fellow journalist, Jim Foley, at the hands of the Islamic State in Iraq and Syria," she said.
Rice also referenced Foley's killing, which she said "shocked the conscience of the world," and said the United States "will continue to use all of the tools at our disposal to see that the remaining American hostages are freed."
"We will continue to work tirelessly on behalf of all Americans who are held overseas so that they can be reunited with their families as well," she said.
The US envoy to the United Nations, Samantha Power, echoed those remarks, saying Curtis, Foley and other journalists "traveled to Syria to shed light on the unspeakable horrors being committed against innocents -- only to become victims of brutal forces unleashed and abetted by the conflict."
Foley's family, meanwhile, prayed for the safety of his fellow hostages as hundreds gathered for a mass to celebrate his life as a witness on the front line.
Before the service, they told AFP that they hoped their son's legacy would be an inspiration to others supporting a free press and an end to suffering in war zones.
"James stood for love and hope," Diane Foley said.
Britain's ambassador to the United States said meanwhile that authorities were "close" to identifying the hooded Islamic State jihadist who beheaded Foley.