The witnesses said the grave, near the Sinjar Technical Institute, contained the remains of 78 women between the ages of 40 and 80 years old, Los Angeles Times reported.
The gruesome discovery was followed by the finding of another grave Sunday 10 miles west of Sinjar believed to contain the bodies of about 50 men. Both graves have yet to be excavated.
Iraqi Kurdish officials said the dead were probably Izadis, members of a religious minority from Sinjar who were the target of a wave of terror by the Takfiri group. The militants seized the town in August 2014 as they swept across northern Iraq from their bases in Syria.