Mr Obama told NBC TV the US would degrade IS, shrink its territory and "defeat them".
He was criticised last month for saying "we don't have a strategy yet" when asked about IS during a briefing.
The US expanded its operations on Sunday, carrying out air strikes on IS for the first time in western Iraq, to defend Iraqi troops at Haditha dam.
Speaking on NBC's Meet the Press, Mr Obama said: "I'm preparing the country to make sure that we deal with a threat from Isil."
IS, also often referred to as Isil or Isis, has taken over large swathes of Iraq and Syria in recent months, declaring the land it holds a "caliphate".
Mr Obama went on: "On Wednesday, I'll make a speech and describe what our game plan's going to be going forward."
He said he would "start going on some offense" against IS.
But the strategy was "not going to be an announcement about US ground troops", he added.