News of Curtis's release emerged just days after the militant group Islamic State posted a video on the Internet showing one of its fighters beheading American journalist James Foley, who was kidnapped in Syria in 2012.
Kerry said Curtis had been held by Nusra Front, al Qaeda's official wing in Syria whose rivalry with Islamic State has fueled war among the insurgents themselves.
The U.N. issued a statement confirming Curtis's release.
"The United Nations can confirm that it facilitated the handover of Peter Theo Curtis. He was handed over to UN peacekeepers in Al Rafid village, Quneitra, the Golan Heights, at 6:40 p.m. (local time) on 24 August 2014. After receiving a medical check-up, Mr. Curtis was handed over to representatives of his government."
Curtis, 45, also goes by the name Theo Padnos, according to his family, who described him as a journalist and a writer.
White House national security adviser Susan Rice said in a statement that Curtis was "safe outside of Syria, and we expect he will be reunited with his family shortly."
A man who answered the phone at the Cambridge, Massachusetts, home of Nancy Curtis, Theo Curtis's mother, said she was not speaking to the media. The man declined to comment, but said the family was very happy over the news of Curtis's release.
Curtis has written two books, both under the names Theo Padnos: "Undercover Muslim: A Journey into Yemen," a memoir of studying at a madrasa, and "My Life Had Stood a Loaded Gun," a memoir about teaching literature to teenage offenders in Vermont.
The video of Foley's beheading posted on Tuesday, together with a threat to kill another American journalist being held hostage, Steven Sotloff, inspired widespread revulsion in the West and a desire to hunt down Foley's killer.
The U.S.-based Committee to Protect Journalists estimates that about 20 journalists are missing in Syria. Many of them are believed to be held by Islamic State.
In a video statement released by Curtis's kidnappers at some point during his captivity, Curtis said he was a journalist from Boston, Al Jazeera reported.
Commenting on his treatment in that video, Curtis said he "had everything" he needed and "everything has been perfect, food, clothing, even friends now." (Reporting by Amena Bakr and Yara Bayoumy in Dubai and Eric Beech in Washington; Additional reporting by Fiona Ortiz; Editing by William Maclean, Angus MacSwan and Leslie Adler)