Members of parliament unanimously approved the motion – which is not binding on the government – by 278 votes to zero.
The vote in the 650-seat lower House of Commons calls on ministers to accept formally that IS actions against Christian, Yazidi and other religious and ethnic minorities in Syria and Iraq constitute genocide.
But Foreign Office junior minister Tobias Ellwood, who has specific responsibility for the Middle East, said it was up to the courts rather than the government to make such a judgement.
“I believe genocide has taken place, but as the prime minister (David Cameron) has said, genocide is a matter of legal rather than political opinion,” Ellwood said.
MPs from all parties urged Britain to use its position as one of the five permanent members of the United Nations Security Council to get the situation referred to the International Criminal Court.