Construction worker Jay Marx repeatedly shouted "John Howard, get out of Iraq. The Bush administration is a sinking ship".
Mr Howard was speaking to journalists outside Blair House, the official residence where he and his wife Janette are staying as guests of President Bush.
Although Marx was at least 20 metres away from Mr Howard , he was clearly distracted the prime minister.
Eventually, secret service agents moved a black van to block his view of Howard.
Marx told journalists he had recognised Mr Howard after seeing Australian flags outside the White House.
"I know that the majority of Australian people oppose the war, I know that John Howard has supported the Bush administration from the get-go, and it's pathetic," he said.
Marx said he happened across the Australian PM by chance.
He was at nearby Lafayette Park for a mothers' rally organised by CODEPINK, a
women's peace group.