The civilians were shot, burned or beheaded, according to the Syrian Observatory for Human Rights’ director Rami Abdel Rahman.
The victims included Alawite Sunni and Ismaili Shia Muslims and the atrocity took place in the village of Mabouja in Hama province during a raid, according to the observatory.
Forces loyal to President Bashar al-Assad regained control of Mabouja after the late-night raid by the IS.
Government soldiers and civilians were reportedly killed in airstrikes on IS positions around the village amid a massive assault by the Sunni extremist group on Hama.
The observatory relies on a network of activists and medics in Syria for its information.