The Syrian Observatory for Human Rights said the jihadists shot dead seven men in Rastan, a town in Homs province of central Syria, "after accusing them of being homosexual".
IS also executed two men and the boy in the town of Hreitan, in the northern province of Aleppo, for the same reason, said Observatory head Rami Abdel Rahman.
He said the executions were carried out in public, but that IS fighters destroyed any cameras that had been used to film the killings.
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IS has used brutal violence to enforce its rule in the so-called "caliphate" it declared in territory straddling Iraq and Syria.
Along with homosexuality, witchcraft and loyalty to President Bashar al-Assad are also considered capital punishment offences in IS-controlled areas.
In previous cases, IS has beheaded alleged homosexuals or thrown them from rooftops.
According to the Observatory, IS has executed more than 3,000 people, including at least 1,800 civilians, since it announced its caliphate in June 2014.