They said soldiers early on Thursday broke into a house where Muhammad Alasi, 24, a local leader of al-Aqsa Martyrs Brigades, was talking to the reporter.
Alasi tried to flee, but soldiers shot and killed him and took his body, the witnesses said.
The Israeli military had no immediate comment.
The witnesses did not know which news outlet the reporter represented, giving her name only as Annie.
Palestinian police said the reporter, in her 60s and living in Nablus for a few years, was being questioned at her home and was in shock.
The al-Aqsa Martyrs Brigades is linked to the ruling Fatah Party of Palestinian leader Mahmoud Abbas.
Nablus, the largest city in the West Bank, is a main centre of resistance activity. Israeli forces carry out frequent raids there.