Environmental campaigners have been aiming to prevent the building of the pipeline, which will transport oil reserves from Canada through the heart of the U.S. to the Gulf of Mexico.
On Thursday, Obama announced the plan would be shelved until after the 2012 presidential election to allow for a new review of the proposal - and Redford has recorded a short message thanking the U.S. leader for making the "tough" decision.
In the clip, posted online, the Out of Africa star says, "Sometimes a decision can be both easy and tough. It's easy because you know what choice is right and it's tough because of the pressure people apply to influence your thinking.
"For months now, big oil has done everything it could to pressure President Obama to approve the Keystone XL pipeline. This pipeline would have carried the dirtiest oil in the world - tar sands - right through the American heartland, threatening our drinking water and putting foreign corporate profits ahead of America's wellbeing.
"This pipeline would have tied our energy future to the destructive ways of the past. It would not have met America's energy needs, nor would it have created substantial permanent jobs. President Obama didn't cave to the pressure. He weighed the facts and he looked at the kind of future he promised when he ran for President. He made a tough decision by doing what was right for America, for our health, and for our clean energy future.
"So I join many others, Mr. President, in saying thank you. Thank you for standing up to big oil and for standing with your fellow Americans."