The Syrian Observatory for Human Rights said 32 civilians were among 91 people executed for "crimes" in the jihadist group's self-proclaimed caliphate between July 29 and August 29.
The toll included IS members, rebel fighters and forces loyal to President Bashar al-Assad.
Witchcraft, homosexual acts and working with the US-led coalition fighting IS in Iraq and Syria are among "crimes" punishable by death for the group.
Since it announced its "caliphate" in June 2014, IS has swept across Syria, seizing swathes of land in central Hama and Homs provinces, Deir Ezzor and Hasakeh in the east, and the north's Raqa and Aleppo.
The number includes 1,841 civilians.
IS has become notorious for its gruesome displays of violence, including beheadings, stoning and pushing victims off rooftops.