The US State Department is still studying whether to go ahead with the Keystone XL project.
The Democratic presidential candidate has previously avoided taking a position on the matter.
Now, at a campaign event in Iowa, Hillary Clinton said the pipeline would be a “distraction from the important work we have to do to combat climate change”.
“Therefore, I oppose it,” she told supporters.
Until now, former Secretary of State Hillary Clinton had avoiding giving an opinion on the project, saying she did not want to interfere with the Obama administration’s deliberations.
However, Hillary Clinton’s main rival for the Democratic candidacy, Bernie Sanders, has long opposed the scheme and called on her to make her position clear.
The 1,179-mile pipe was first proposed seven years ago, and would transport oil from the oil sands in Alberta, Canada, to the Gulf of Mexico via Nebraska.
Supporters say the project will create jobs and US increase energy security, while opponents say it will only increase carbon emissions.
Three Indian nationals have been arrested and charged over the killing of Sikh separatist leader…
President Joe Biden has urged pro-Palestinian protesters on university campuses to uphold the rule of…
Blue Ivy Carter has joined the voice cast of The Lion King prequel Mufasa: The…
At least five people, including a four-month-old baby, have been killed after dozens of tornadoes…
Harvey Weinstein has been hospitalized just days after his 2020 rape conviction in New York…
Hamas has published a video showing the first proof of life of US and Israeli…