"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.
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 to be used as sex slaves.
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The evidence collected by the COI can serve "as a roadmap for prosecutions," commission member Carla del Ponte told reporters.
Del Ponte, previously a prosecutor at international tribunals for Rwanda and the former Yugoslavia, said she would be able "to prepare an indictment" for genocide against IS commanders based on the testimony in the report.
Commission member Vivit Muntarbhorn said IS fighters of multiple nationalities, possibly including Westerners, could be guilty of genocide.
But definitively naming IS leaders based on witness testimony is complicated because many use pseudonyms, the commission said.
Past moves to refer violations in Syria to the International Criminal Court have stalled in the UN Security Council.
The COI members voiced hope that this case would be different because IS is broadly reviled and does not have political support in the Security Council, unlike Syria's government.