"I think it is imperative that we look at the Keystone pipeline as what I believe it is - a distraction from the important work we have to do to combat climate change, and unfortunately from my perspective one that interferes with our ability to move forward to deal with all the other issues," Clinton said in Des Moines today. “Therefore I oppose it.”
Clinton's announcement comes amid growing calls from voters on the left for her to make her position on the pipeline, which stretches from Canada through Nebraska and to the Gulf Coast, known.
Previously, Clinton had refused to take a stance on the pipeline citing her previous involvement with the issue as Secretary of State. Last week, however, Clinton changed her tune and told voters during a town hall in New Hampshire that she would make her position known "soon" -- saying that while she had wanted to wait for the Obama administration to make a decision on the pipeline first, they were taking too long.
On Wednesday, Clinton reiterated that sentiment.
"I thought this would be decided by now. Therefore I can tell you whether I agreed or I disagreed. But it hasn’t been decided and I feel now I got a responsibility to you and other votes who ask me about this," she said when asked by a young woman to give her position.
Clinton, who said she will be rolling out a North America climate change plan in the coming weeks, came out against the pipeline because, she said, "I don’t think it’s in the best interest of what we need to do to combat climate change."
"I do believe there is a lot of work to be done. We have a lot of pipelines, that are leaking, they need to be repaired, they are dangerous, they are leaking methane, they are at risk of causing damage. So I want to put thousands of Americans to work who are not only going to fix those old pipelines, but also we’ve got railcars transporting oil and I want those railcars, and the rail beds and the tracks they are on to be repaired," Clinton explained.
"So the grid has to be updated in order for it to be more accessible to more renewable energy. We have a lot of work to do. A lot more jobs from my perspective on a North American clean energy agenda than you will ever get from just one pipeline crossing the border," she added.
Clinton’s Democratic presidential rivals Bernie Sanders and Martin O’Malley have both been on the record in opposition to the Keystone pipeline for months, and have often bring it up as a point of contrast with Clinton.
Both candidates released statements responding to Clinton's announcement.
“As a senator who has vigorously opposed the Keystone pipeline from the beginning, I am glad that Secretary Clinton finally has made a decision and I welcome her opposition to the pipeline," Sanders said in a statement. "Clearly it would be absurd to encourage the extraction and transportation of some of the dirtiest fossil fuel on the planet."
Former Maryland Governor O'Malley slammed Clinton for taking so long to take a position.
"On issue after issue--marriage equality, drivers licenses for undocumented immigrants, children fleeing violence in Central America, the Syrian refugee crisis, and now the Keystone Pipeline, Secretary Clinton has followed--not forged--public opinion. Leadership is about stating where you stand on critical issues, regardless of how they poll or focus group. " his statement read.
ABC News' MaryAlice Parks contributed to this report.