With 93 percent of the ballots counted from nearly 34 million eligible voters, Fernandez received 47.9 percent of votes. The Peronist leader will serve as president until 2023.
Peronist candidate Alberto Fernandez swept to a first-round triumph in Argentina's presidential election Sunday, official results showed, bringing to an end the crisis-plagued rule of market-friendly incumbent Mauricio Macri.
Fernandez, a 60-year-old law professor, had nearly 48 percent of the votes — crossing the threshold for outright victory — after 94 percent of the votes had been counted, with centre-right incumbent Mauricio Macri trailing at 40.53 percent.
To win outright, Fernandez required 45 percent, or 40 percent with a 10 point margin over his nearest rival.
His win also caps a remarkable political comeback for his running mate, ex-president Cristina Kirchner, who will be his vice-president.
Thousands of ecstatic Fernandez supporters cheered and danced outside his Frente de Todos party headquarters in Buenos Aires.
"It's a great day for Argentina," a smiling Fernandez told reporters after exit polls and his own party's tallies had given him victory.
Macri, 60, conceded with around 80 percent of the vote counted and pledged "a healthy and constructive opposition."
He said he had spoken by phone to Fernandez and had "invited him to breakfast tomorrow at the Casa Rosada because he has to start a period of orderly transition that will bring tranquility to Argentines," Macri told his supporters.
Fernandez, who assumes power on December 10, confirmed in a victory speech that he would meet Macri on Monday morning to collaborate on the transition.
Macri, whose popularity has fallen sharply in the last year as Argentina battled recession and market turmoil, said after he voted that competing "visions of the future" were at stake in the polls.
On a night of disappointment for the centre-right, there was some consolation when the candidate for Macri's Together for Change coalition, Horacio Rodriguez, won the mayorship of the city of Buenos Aires. He beat the leftist candidate Matias Lammens by some distance.
The interior ministry said turnout in Sunday's general election was over 80 percent after a campaign dominated by the crippling economic crisis affecting Latin America's second-biggest economy.
Macri had called for a massive turnout, which analysts saw as his main hope of closing a large opinion poll deficit on Fernandez and forcing a second round.
Fernandez vowed to end sharp divisions between his Peronist movement and supporters of the business-friendly incumbent.
"The days of 'Us' and 'Them' are over," the mustachioed leftist leader said after voting in the swanky Puerto Madero neighbourhood of Buenos Aires. "We are in an enormous crisis. Everyone has to take responsibility for what's ahead."
The election comes amid high tensions in the region, with massive protests in neighbouring Chile and Bolivia, as well as recent unrest over inequality in Ecuador.
Voter Maria Marta Rosauer, 54, said she would give Macri "another vote of confidence."
"I voted with the conviction and the certainty that he did things well and that he could have done better, but he needed time," she said.
"Nobody can put a country on its feet in four years, after how he found it. We opened our doors to the world after many years of being almost forgotten," she said, referring to the years when Argentina was a market pariah following a 2001 default.
There are "two models of government at stake here. Alberto and Cristina represent greater equity," said another voter, Liliana, a 64-year-old architect in the capital. "I'm excited to see the end of a country that only benefits a small group."
The populist Kirchner, 66, who is facing trial in one of several graft cases stemming from her time in office, voted in the southern city of Rio Gallegos before flying to Buenos Aires to join celebrations.
Kirchner has long been a polarizing figure who succeeded her husband Nestor Kirchner as president in 2007 and remained in power until 2015.
The return to power of protectionist Peronists comes amid a lengthy recession and a debt crunch, raising market fears of a possible default on a $57 billion IMF loan.
The peso fell 5.86 percent in the week before the elections, and the week ended with the dollar at 65 pesos.
"The markets will be negative" in their reaction to a Fernandez triumph on Monday, said Nicolas Saldias, a senior researcher at the Wilson Center.
"It won't be as brutal as in August (after the primary result), but people are taking their money out of the country, out of the banks."
"A lot depends on whether he shows his capacity to compromise.
"Macri is the president but Fernandez has the power. There has to be some signal that they are working together."
Fernandez has insisted his government would not default but rather seek to renegotiate the terms of the loan, and sought to reassure voters that their bank deposits would be safe under his administration.
Since Fernandez's crushing victory in August primaries, which made him the favourite for the presidency, Argentine savers have withdrawn around $12 billion from their accounts.
Macri blamed Argentina's economic problems on previous Peronist governments under Kirchner (2007-15) and her late husband Nestor Kirchner (2003-07).
The poverty rate has risen to more than 35 percent, inflation for the year to September was at almost 38 percent, while the peso has depreciated 70 percent since January 2018.
Voters were also electing half of the Chamber of Deputies, a third of the Senate, and the governor of Buenos Aires province.