The Syrian Observatory for Human Rights, a Britain-based NGO, said it had confirmed that at least 27 Yazidi women had been sold for around USD 1,000 each to IS fighters.
The group said it was aware that some 300 Yazidi women had been kidnapped and transported to Syria by the jihadists, but it had so far documented the sale into marriage of 27.
"In recent weeks, some 300 women and girls of the Yazidi faith who were abducted in Iraq have been distributed as spoils of war to fighters from the Islamic State," a statement said.
The group said it had documented several cases in which the fighters then sold the women as brides for USD 1,000 each to other IS members after forcing them to convert to Islam.
"The Observatory documented at least 27 cases of those being sold into marriage by Islamic State members in the northeast of Aleppo province, and parts of Raqa and Hassakeh province," the NGO said.
It added that some Syrian Arabs and Kurds had tried to buy some of the women in a bid to set them free, but they were only being sold to IS members.
The Observatory said it was unclear what had happened to the rest of the 300 women, and strongly denounced the "sale of these women who are being treated as though they are objects to buy and sell."
Both UN officials and Yazidis fleeing IS advances in Iraq have said fighters kidnapped women to be sold into forced marriages.
UN religious right monitor Heiner Beilefeldt warned earlier this month of reports of women being executed and kidnapped by IS militants.
"We have reports of women being executed and unverified reports that strongly suggest that hundreds of women and children have been kidnapped -- many of the teenagers have been sexually assaulted, and women have been assigned or sold to 'IS' fighters," she said.
Yazidis, a Kurdish-speaking minority who follow an ancient faith rooted in Zoroastrianism, are dubbed "devil worshippers" by IS militants because of their unorthodox blend of beliefs and practices.
The IS emerged from the one-time Iraqi affiliate of Al-Qaeda but has since broken with that group and espouses an interpretation of Islam that has been widely rejected.