The Syrian Observatory for Human Rights said they were thought to be members of Shaitat tribe that fought the jihadist group in Deir al-Zour province in the summer.
The mass grave was discovered after members of the tribe were allowed to return to their homes by IS, it added.
Last month, the UN said it had received reports of a massacre there in August.
The discovery brings the number of members of the Shaitat tribe killed during the jihadists’ summer advance in Deir al-Zou province near Iraq to more than 900, the Syrian Observatory for Human Rights said.
It said the “vast majority” were civilians, and that many of them were executed in cold blood after the tribe rose up against the IS after it had driven out rival jihadists and rebels from the area.