Javier Milei, a libertarian political outsider who pledged to flatten Argentina’s political establishment, was elected president in a major shift for a country in economic straits.

BUENOS AIRES—Javier Milei, a libertarian political outsider who pledged to flatten Argentina’s political establishment, won the presidency Sunday by an overwhelming margin in a major shift for a country buffeted by one of the world’s highest rates of inflation and mounting poverty after years of populist rule.

Milei, a 53-year-old congressman, took nearly 56% of the vote to 44% for Economy Minister Sergio Massa with 98% of the ballots counted in the runoff election, the National Electoral Directorate said.