With nearly all the votes counted, Duque secured 54 percent on Sunday, comfortably ahead of his leftist rival Gustavo Petro, at 42 percent.
About four percent of the voters submitted spoiled ballots, according to electoral officials.
In the capital, Bogota, enthusiastic Duque supporters honked car horns even before final results were announced.
Polls opened at 13:00 GMT and closed at 21:00 GMT after a day of peaceful voting.
In May’s first round, Duque came first with nearly 40 percent of the vote, while Petro received 25 percent.
The second round of voting was seen by some as a referendum on the controversial peace deal with the Revolutionary Armed Forces of Colombia (FARC) in 2016.
Duque, 41, vowed to overhaul the pact while Petro, a 58-year-old one-time rebel with the now defunct M-19, supported the fragile peace process.
A pro-business candidate, Duque promised to favour entrepreneurs and cut corporate taxes.
He also warned that leftist rule could turn traditionally conservative Colombia – Latin America’s fourth-biggest economy – into a new Venezuela, the crisis-hit neighbouring country led by socialist Nicolas Maduro.
He is supported by former President Alvaro Uribe, one of the peace accord’s fiercest critics for its perceived soft judicial treatment of “FARC terrorists”.
For his part, Petro – a former Bogota mayor – pledged to take on political elites, redistribute land to the poor and gradually eliminate the need for oil and coal in
Duque will succeed President Juan Manuel Santos, whose efforts to end the war with the FARC which killed 220,000 people and displaced nearly seven million brought him the Nobel Peace Prize.
Duque’s victory means he will be Colombia’s youngest president since 1872. His swearing-in ceremony will take place in August.
|Supporters of Duque celebrate in the Colombian capital [Andres Stapff/Reuters]|
SOURCE: Al Jazeera and news agencies