The Pentagon said American aircraft attacked artillery that was being used against Kurdish forces defending the northern city of Irbil.
President Barack Obama authorized air strikes on Thursday, but said he would not send US troops back to Iraq.
The Sunni Muslim group IS, formerly known as Isis, now has control of large swathes of Iraq and Syria.
In June, IS took control of the city of Mosul. Earlier this week, its fighters seized Qaraqosh, Iraq’s biggest Christian town.
The advance of IS also forced tens of thousands of people from the Yazidi community – another minority group in northern Iraq – to leave their homes and seek shelter on a nearby mountain.
According to the Pentagon statement, two F/A-18 aircraft dropped 500-pound laser-guided bombs on mobile artillery near Irbil.
Pentagon press secretary Rear Admiral John Kirby said IS had been using the artillery to shell Kurdish forces defending Irbil, where US personnel are based.
The air strike is the first time the US has been directly involved in a military operation in Iraq since American troops withdrew in late 2011.
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