The election pits Peronist Economy Minister Sergio Massa, at the helm for the country's worst economic crisis in two decades, against radical libertarian outsider Javier Milei, the slight favourite in pre-vote opinion polls.
Milei is pledging economic shock therapy, from shutting the central bank to ditching the peso and slashing spending, potentially painful reforms that have resonated with voters angry at the economic malaise, but which have sparked fears of austerity in others.
With many Argentines unconvinced by either candidate, some have characterised the election as a choice of the "lesser evil": fear of Milei's painful economic medicine or anger at Massa over the economic crisis. Many Argentines say they will not vote at all.
In the first-round vote in October, Massa won 36.7 per cent of the votes compared with some 30 per cent for Milei. The libertarian has since won public backing from third-place finisher Patricia Bullrich, though it is not certain all her votes will shift to him.
Whoever wins, it will shake up Argentina's political landscape, its economic roadmap, trade in grains, lithium and hydrocarbons, and its ties with China, the United States, Brazil and others.
"None of the candidates gives me certainty about the future," said Josefina Valente, a 63-year-old retiree as she voted in Buenos Aires on Sunday morning.
"I come to vote out of obligation so that once and for all we have a change in the country."
The story of the race so far has been the rise of 53-year-old economist and former TV pundit Milei, a lightning rod for voter anger who has threatened to detonate the status quo and tear down what he calls a "caste" of the political elite.
Milei on Sunday decried a "campaign of fear" against him, but expressed confidence.
"Now we'll let the polls speak," Milei said after voting in the city of Buenos Aires.
"Let's hope that tomorrow there will be more hope and an end to the decay."
After voting in the province of Buenos Aires, Massa told reporters: "This is an extremely important election that defines our country's direction in the next four years."
The winner is set to take office on December 10 and would replace outgoing centre-left Peronist President Alberto Fernandez.
