One of Trump's first acts after taking office in January was to give a conditional go-ahead for the controversial cross-border project, which Obama had put on hold.
Trump was due to speak about the decision Friday at 10:15 am (1415 GMT), his spokesman Sean Spicer said on Twitter.
After a new US review of the project, Under Secretary of State Thomas Shannon issued the presidential permit, concluding that it would "serve the national interest," the State Department said Friday.
The 1,180-mile (1,900-kilometer) conduit would carry oil from Canada's tar sands to US Gulf Coast refineries, with some 870 miles winding through the United States.
Trump repeatedly asserted during the US presidential campaign that he would approve the pipeline.
TransCanada thanked the US administration for reviewing and approving the delayed project.
"This is a significant milestone for the Keystone XL project," said TransCanada president and chief executive officer Russ Girling.
The company has a total of $15 billion in investment in oil and natural gas "that will create thousands of well-paying jobs and generate substantial economic benefits across the US."
But TransCanada still will need to work with authorities and residents to obtain the necessary permits and approvals to advance the Keystone XL project to construction in Nebraska, Montana and South Dakota, the company said.
Protestors supporting the Standing Rock Sioux Native American tribe for many months blocked completion of a section of the Dakota Access pipeline in North Dakota, until the Trump administration overrode their environmental concerns and approved the construction.
Canada is the world's sixth-largest oil producer thanks to the Alberta tar sands, which produce some of the "dirtiest" crude in the world.
Unlike traditional crude oil which gushes from a well, tar sand oil must be dug up and essentially melted with steaming hot water before it can be refined. It results in huge lakes of polluted water and the strip-mining of millions of acres of once-pristine boreal forests.
Environmentalists say that tar sand oil contains a harmful and corrosive component -- bitumen -- which makes pipeline ruptures or leaks more likely and carries greater health and safety risks.