Michael Leighton Phillips, 24, was reported missing by his project manager earlier on Wednesday.
An official of the Fatah movement, Jamal Tirawi, said fighters of al-Aqsa Martyrs Brigades had rescued Phillips from his captors.
"There were exchanges of gunfire with the hostage-takers, but the student was rescued and handed over to Ghassan al-Shaka, a member of the executive committee of the Palestinian Liberation Organisation, at whose house he is now," he said.
A group of armed men had earlier summoned reporters in the Gaza Strip and claimed to represent the group holding Phillips.
"Our demands are the immediate release of all imprisoned women and minors," a masked member of the group Ansar al-Sunna said.
He said Phillips was abducted to "confront the American war on Islam".
Samah Atut, manager of Project Hope, said she received a call saying Phillips had been kidnapped. She refused to give further details.
Atut said Phillips was working with Project Hope in refugee camps near the West Bank city of Nablus, teaching English. She said Phillips came from Los Angeles and lived alone in a Nablus neighbourhood.
Foreigners have been kidnapped and held briefly in Nablus in the past, then released unharmed.
The most recent case was in February during the issue over the Danish caricatures of the Prophet Muhammad, when a German volunteer was abducted and held for several hours.