Mohamed Al-Namli, a member of Al-Khoms appeal court, was found dead on Tuesday near Al-Harawa town with signs of torture on his body, the organisation said, holding IS affiliates responsible for the murder, Xinhua reported.
Al-Namli was kidnapped by an unidentified armed group, which is most likely to be affiliated with the IS, in the city of Sirte, 450 km east of the capital Tripoli, when he was passing through the city's western gate.
The city of Sirte is controlled by militants loyal to the IS after Libya Dawn militias evacuated the city.
Judicial officials in Libya, including consultants and judges in Tripoli, Benghazi and Derna, have been targeted in numerous cases.
Libya, a major oil producer in North Africa, has been witnessing a frayed political process after former leader Muammar Gaddafi was toppled during the 2011 political turmoil.
The country is now deadlocked in a dogfight between the pro-secular army and Islamist militants, which has led to a security vacuum for homegrown extremism to brew.