Redford, who is an Academy Award winner and long-time environmentalist, argued that the proposed pipeline may threaten the environmental health of America's heartland and compromise a major water supply.
Countering the argument that the pipeline would actually create permanent jobs, Redford said, "By deepening our reliance on oil, the pipeline would be a job killer."
Redford wrote last month in The Huffington Post:
Here's a better idea. Let's build the next generation of energy efficient cars, homes and workplaces. Let's develop wind, solar and other cleaner, safer, more sustainable sources of power and fuel. Let's invest in high-speed rail and smart communities that give us better transportation options.
Terry Cunha, a spokesman for the pipeline company TransCanada, told the Canadian Press, Redford is "entitled to his opinion." Cunha added, "Under any scenario, the need for crude oil as the primary transportation fuel will remain for decades. The U.S. has an important choice of receiving more oil from its most secure, stable and reliable trade partner, Canada, or to continue to import from less stable locations that do not share the interest and values of Americans."
The Obama administration is expected to make a decision on the pipeline by the end of the year.
View some of the celebrities who have spoken out against the pipeline below: