"For the sake of the Syrian people, the time has come for President Assad to step aside," Obama said in a statement as he announced another round of sanctions on Syria.
Noting that the future of Syria must be determined by its people, Obama said but Assad is standing in their way.
"His calls for dialogue and reform have rung hollow while he is imprisoning, torturing, and slaughtering his own people. We have consistently said that President Assad must lead a democratic transition or get out of the way. He has not led," Obama said.
Obama said the US has been inspired by the Syrian peoples'' pursuit of a peaceful transition to democracy.
"They have braved ferocious brutality at the hands of their government. They have spoken with their peaceful marches, their silent shaming of the Syrian regime, and their courageous persistence in the face of brutality day after day, week after week," he said.
But the Syrian government has responded with a sustained onslaught, Obama said in the first explicit US call for Assad to resign.
The military crackdown against anti-government protesters since March said to have left about 2,000 civilian dead and many injured. 45-year-old Assad was elected as president in 2000 and 2007, but was unopposed each time.