Islamist militants kidnapped and beheaded a young shepherd in mountains in Tunisia near the border with Algeria on Sunday, security sources and residents said.
Troops found the body of Okba Dhouibi after he had been seized by militants, the sources said.
Prime Minister Hichem Mechichi said that act would not go unpunished.
Tunisian forces are carrying out operations in the Kasserine and Saloum mountain range to flush out Ajned Kilafha militants allied to Islamic State.
Militans linked to Al Qaeda and IS groups has been sheltering for years in the desolate, hilly terrain along a stretch of the border with Algeria and sometimes clash with security forces there, but the threat is regarded as having been contained.