The beheadings took place in the eastern Deir al-Zor province this week, said Rami Abdulrahman, head of the Syrian Observatory for Human Rights, which tracks the conflict using sources on the ground.
One of the women was beheaded along with her husband in Deir al-Zor city. In al-Mayadeen city to the southeast, the group beheaded another woman and her husband. All of them were accused of sorcery, the monitor said.
Islamic State has beheaded local and foreign men in Syria, including enemy combatants, aid workers and journalists as well as people it has deemed to be violating its hardline interpretation of Islamic law.