Shocking video showing the captive exploding into a ball of flames after being shot with a rocket-propelled grenade that is designed to take out tanks and aircraft.
The poor victims is accused he was fighting against the terror group in Syria.
They then subject him to more humiliation by taking it in turns to kick the corpse while dancing and cheering in celebration.
The video was shared online by anti-ISIS activist group "Syria Is Being Slaughtered Silently", which said the prisoner was accused of fighting against ISIS.
The man, named by the group as Ibrahim Shraideh, was said to be a member of the Al Shaitat tribe from the town of Abu Hamam in Syria's Deir ez-Zor province.
It said the militants were from Tunisia and Morocco.
The 80,000-strong Shaitat clan rose up against the terror group after its offensive across Iraq and Syria last summer.
They later agreed a peace deal, but this crumbled when three members of the tribe were arrested for reportedly refusing to collect taxes for the caliphate.
Nearly 1,000 Shaitats are understood to have been masacred last year alone, including 230 who were found in a mass grave.