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 Islamic State 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.
According to the Britain-based Observatory, the latest monthly toll has brought to 3,156 people the number of IS executions in Syria since the caliphate was announced.
The number includes 1,841 civilians.
IS has become notorious for its gruesome displays of violence, often shown in videos posted on the Internet, including beheadings, stoning and pushing victims off rooftops.
The US-led coalition has been bombarding IS in Syria since September 2014.
On Saturday, eight IS fighters were killed in a coalition air raid on Raqa, the de facto “capital” of their caliphate.
Further west, in Aleppo province, IS has been fiercely attacking rebel groups and its jihadist rivals, Al-Qaeda affiliate Al-Nusra Front.
Late Saturday, an IS car bomb on Tal Rifaat, in Aleppo province, left 11 fighters and eight civilians dead.
Tal Rifaat lies along a key rebel supply route from the Turkish border to the north.
The town is located near Marea, the main rebel stronghold in Aleppo that has been under fierce IS attack for months and is now completely encircled by the jihadists.