"We have detained al-Ghoul and he is being questioned over his activities against us and cooperation with the Fayad government," a spokesman for the Hamas-run Interior Ministry, Ehab al-Ghsain, said.
Media earlier reported that the high-ranking Fatah official, who was in Gaza to attend the funeral of his mother-in-law, had been kidnapped by unknown gunmen.
Hamas is accusing al-Ghoul of failing to pay the salaries of thousands of Gaza civil servants, who cooperated with Hamas.
Al-Ghoul frequently criticizes the fundamentalists in his newspaper columns.
Islamic group Hamas seized control of the Gaza Strip from the pro-presidential Fatah movement in June after months of infighting.