It is in contrast to her stand in the 2008 campaign, during which Clinton defended her "Yes" vote for the Iraq war as a way to give the then President George W. Bush authority to deal with Iraq.
"It did not turn out the way that I had thought it would, based on what he had said," she said of the Iraq war during a town hall event aired live on the ABC "Good Morning America" programme Thursday morning.
Hillary Clinton joined "Good Morning America" two days after winning the New York Democratic primary. She took questions from the two hosts of ABC and questions from the audience.
"And I regret that. And I said that it was a mistake and, obviously, is something that I wish hadn't turned out the way it did." she added.
During the campaign, the Democratic candidate has often been criticized for her vote in support of the Iraq war when she served as the US Senator from New York State.
Her Democratic nomination rival, Bernie Sanders, said earlier that "the disastrous invasion of Iraq, something that I strongly opposed, has unraveled the region completely and led to the rise of al-Qaeda and to (the emergence of) IS."
"I think that (the Iraq war) was one of the worst foreign policy blunders in the history of the United States," Sanders said.
Apart from the Iraq war, Clinton has kept giving different answers for what she thinks is her greatest regret in politics, including not overhauling the healthcare system earlier and the 2012 attacks killing four Americans in Benghazi, Libya.