The review found only a 4 percent risk of a spill larger than 1,000 gallons. The report said flooding and third-party damage could increase the risk of a spill.
"This final review puts to rest any credible concerns about the pipeline's potential negative impact on the environment," Jack Gerard, president of the American Petroleum Institute, said in a statement.
Keystone XL runs for 1,179 miles from Hardisty, Alberta, to Nebraska, where it would connect with an existing pipeline to Cushing, Okla. It would carry oil extracted from Canadian oil sands to a terminal on the Gulf of Mexico.
The State Department review is only one step in the process of approval, CNN said. Eight U.S. agencies will review it and offer feedback, then Secretary of State John Kerry will make a final recommendation to the White House.
President Obama will make the final decision.
Supporters say the pipeline would reduce imports from other more volatile oil-producing areas and create jobs. Opponents have expressed concern about the potential harm to the environment and question whether many long-term jobs would be created.