President Trump on Tuesday gave a conditional go-ahead for the project, which was put on hold over environmental concerns.
Trump on Tuesday gave a conditional go-ahead for the project, which was put on hold by former president Barack Obama over environmental concerns.
Calgary-based TransCanada said in a statement it had filed a "presidential permit application" with the US State Department for approval of the project.
The 1,180-mile (1,900-kilometer) pipeline would carry oil from Canadian 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.
"This privately funded infrastructure project will help meet America's growing energy needs as well as create tens of thousands of well-paying jobs," TransCanada CEO Russ Girling said in a statement.
The statement added that the project would add $3.4 billion to the US economy.
TransCanada must now wait while the US conducts a new study of the Keystone XL project. But between Trump's conditional go-ahead and his nomination of Rex Tillerson, the former CEO of ExxonMobil, as secretary of state, the project is likely to be approved.
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 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.
TransCanada says that buried pipelines are far safer for transporting oil than ships or trains, and that bringing another 830,000 barrels of oil a day from friendly, neighboring Canada would reduce US dependence on the Middle East and Venezuela by up to 40 percent.
Canadian Prime Minister Justin Trudeau, who supports building the pipeline, recently said he wants to gradually halt exploration of the oil sands and transition away from fossil fuel.