The man was killed in a public square in the town of Sulouk on Monday in front of a crowd that included children, the British-based Observatory for Human Rights said.
Rights groups say Islamic State has beheaded and stoned to death many people in areas it controls in Syria and Iraq for actions they see as violating their interpretation of Islamic law, such as adultery, stealing and blasphemy. They have also killed a smaller number of foreign captives.
A statement posted on a jihadist website said the man admitted to blasphemy before being killed in the countryside of Raqqa province, which the hardline group controls.
The website included images of crowds at the square. One photo showed a blindfolded man kneeling with his head on a wooden block as a masked man in black raised a sword over his neck.
The Observatory, which tracks the conflict using sources on the ground, said last month that Islamic State had killed 1,432 Syrians off the battlefield since the end of June when it declared a caliphate in the territory under its control.
The group, which is being targeted by U.S.-led strikes in Iraq and Syria, has often displayed bodies in public after the killings. -Reuters