Secretary of State John Kerry charges the group with “crimes against humanity” for targeting Christians, Yazidis and Shia Muslims.
The US has declared that the Islamic State group’s slaughter of Christians, Yazidis and Shia Muslims amounts to genocide.
Secretary of State John Kerry’s proclamation changes little, creating no legal obligation for the US to take any additional action.
But he said he hoped the victimised groups would take some comfort in knowing the US recognises the crimes against them.
“Daesh (IS) is genocidal by self-proclamation, by ideology and by actions, in what it says, what it believes and what it does,” Mr Kerry said.
“Daesh is also responsible for crimes against humanity and ethnic cleansing directed at these same groups,” he added.
The declaration by America’s top diplomat met a congressional deadline for a decision on the matter.
Lawmakers had demanded Washington acknowledge the jihadist group is seeking to exterminate religious minorities.
The US House of Representatives passed a non-binding resolution this week condemning IS killings as genocide.
Representative Jeff Fortenberry, the author of the House measure, commended the Secretary of State’s proclamation.
“The United States has now spoken with clarity and moral authority,” the Nebraska lawmaker said.
“I sincerely hope that the genocide designation will raise international consciousness, end the scandal of silence.”
It is only the second time a US administration has declared genocide was being committed during an ongoing conflict.
The first was in 2004, when then-Secretary of State Colin Powell determined atrocities in Sudan’s Darfur region constituted genocide.
Mr Powell reached that decision amid intense lobbying from human rights groups, but only after lawyers advised him it would not obligate the US to act.
Groups including the Catholic organisation Knights of Columbus had released reports last week documenting evidence which it claimed showed the legal standard for genocide had been met.
In March last year, UN investigators said the self-proclaimed caliphate was trying to wipe out the Yazidis.
The Simon Wiesenthal Center, which chronicles past ethnic cleansing and campaigns against bigotry, welcomed the decision to name Christians and Yazidis as victims.
“We reiterate our call that the US put these two groups at the front of the line for consideration for immigration to our country and to redouble our efforts to destroy ISIS,” it said.
The US has been carrying out military strikes since 2014 against IS.
American air strikes on the jihadists around Sinjar were credited with helping rescue tens of thousands of Yazidis, members of a pre-Islamic religious minority.
The UN Convention defines genocide as “acts committed with intent to destroy, in whole or in part, a national, ethnic, racial or religious group”.