IS had claimed in January 2015 to have executed blogger Sofiene Chourabi and photographer Nadhir Ktari but later the same year the Tunisian government said it had evidence the pair were still alive.
The prisoner, identified as Libyan Abderrazek Nassef Abderrazek Ali, told the Al-Hadath Al-Libya channel that he had witnessed the pair being executed in a forest outside the eastern town of Derna, which was then under IS control.
He said the two journalists had been captured at an IS roadblock between the towns of Ajdabiya and Labraq and then taken to Derna.
Some 3,000 Tunisians have joined the ranks of jihadist groups fighting in Libya, as well as in Syria and Iraq, according to Tunisian officials. The United Nations puts the figure at 5,000.
Also Read
Al-Hadath Al-Libya is close to eastern Libya's military strongman Marshal Khalifa Haftar.
The channel said that the suspect, who was shown shaven-headed and wearing an orange jumpsuit, was being held by Haftar's forces.
Relatives of the two journalists conducted a long campaign for information from the Tunisian government on the fate of their loved ones.
Disclaimer: No Business Standard Journalist was involved in creation of this content