Santos, a former defense minister, and former Bogota Mayor Antanas Mockus will be in a June 20 runoff unless Mockus drops out, the Los Angeles Times reported Monday.
After Sunday's votes were counted, Santos took 46.6 percent to 21.5 percent for Mockus, officials said.
Santos, front-runner for much of the campaign, was a defense minister during President Alvaro Uribe's successful battles against leftist guerrillas. He capitalized on his relationship with the president, pledging to carry on security gains that helped improve Colombia during the last decade.
"This was clearly a vote for continuity," Cynthia Arnson, director of the Latin American program at the Woodrow Wilson International Center for Scholars in Washington, told the Times.