"Genocide has occurred and is ongoing," Paulo Pinheiro, head of the UN Commission of Inquiry (COI) for Syria said in a statement.
"ISIS has subjected every Yazidi woman, child or man that it has captured to the most horrific of atrocities," he added, using another acronym for the jihadist group.
The Yazidis are neither Muslims nor Arabs and follow a unique faith despised by IS.
The Kurdish-speaking minority is mostly based around Sinjar mountain in northern Iraq.
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In 2014, IS jihadists massacred Yazidis in Sinjar, forcing tens of thousands of them to flee, and capturing thousands of girls and women as spoils of war.
The UN warned last year that the group appeared to be committing genocide against the Yazidis, but the COI's report "They came to destroy: ISIS Crimes against the Yazidis", published Thursday, was more conclusive.
Based partly on interviews with survivors, the report found that IS "sought to erase the Yazidis through killings, sexual slavery, enslavement, torture and inhuman and degrading treatment."
The group was holding Yazidis in conditions "that bring about a slow death," and was transferring Yazidi children from their families to live with IS fighters, "cutting them off from beliefs and practices of their own religious community," the report said.
It showed that men and boys over 12 were separated from their families, and those who refused to convert to Islam were killed, while thousands of women and girls, some as young as nine, were sold in slave markets.
The report said some 3,200 Yazidi women and children were currently being held by IS, mainly in Syria, where the females were used as sex slaves and the young boys were being indoctrinated and trained as fighters.