The Syrian Observatory for Human Rights said the men were executed and their bodies hung on makeshift crucifixes in Albu Kamal in the eastern province of Deir Ezzor.
"The men surrendered in Albu Kamal because the Islamic State had offered amnesty to people who fought them if they turned themselves in," Observatory director Rami Abdel Rahman told AFP.
Instead, he said, the eight opposition fighters were beheaded and then hung from crosses in a method often employed by the jihadist group.
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The monitor, which relies on a large network of sources on the ground in Syria, said the men had belonged to a group that had fought against both the Syrian regime and the Islamic State.
Meanwhile, in the city of Deir Ezzor, the provincial capital, IS jihadists decapitated another three men, also hanging their corpses from crosses, the Observatory said.
The group said it was unclear when the executions took place, adding that two of the men were accused of collaboration with the Syrian regime and the third of fighting against the Islamic State.
IS has declared a so-called Islamic "caliphate" in the territory it controls in Syria and Iraq, imposing its extreme interpretation of Islam and executing opponents.