"The United States strongly condemns the barbaric murder of UK citizen David Haines by the terrorist group ISIL. Our hearts go out to the family of Mr Haines and to the people of the United Kingdom," Obama said in a statement.
The IS extremists released a video showing the beheading of aid worker Haines, who was abducted in Syria last year.
British Prime Minister David Cameron confirmed Haines' death in a statement late yesterday after the British Foreign Office had said earlier that it was "working urgently to verify the video. He condemned his slaying as "an act of pure evil".
"The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve," said the US President after the ISIL posted a video on social media about the killing of the British citizen.
"We will work with the United Kingdom and a broad coalition of nations from the region and around the world to bring the perpetrators of this outrageous act to justice, and to degrade and destroy this threat to the people of our countries, the region and the world," Obama said.
In the ISIL video released yesterday, a masked person was shown beheading Haines and threatened that another British national would soon be beheaded.
Islamic State militants recently beheaded two American journalists, James Foley and Steven Sotloff, and posted the videos online after the US began launching airstrikes in Iraq.