Far-right libertarian outsider Javier Milei has won Argentina's presidential run-off poll, according to provisional results.
Mr Milei's rival, economy minister Sergio Massa, called him to concede.
Former US President Donald Trump congratulated Mr Milei on his victory, saying he would "Make Argentina Great Again!".
The election comes at a difficult time for Argentina with rising inflation and an economy in crisis.
Mr Milei's proposals, which included "blowing up" the central bank, won support with voters desperate for change.
He confounded pollsters by easily defeating the candidate of the governing Peronists by more than 10 points, provisional results suggest.
With almost 90% of votes counted, Mr Milei had nearly 56% of the vote compared with 44% for Mr Massa.
In his victory speech to supporters in Buenos Aires, Mr Milei said: "Today begins the reconstruction of Argentina. Today begins the end of Argentina's decline.
"The model of decadence has come to an end. There is no way back.
"Argentina will return to its place in the world that it should never have lost. We are going to work shoulder-to-shoulder with all nations of the free world, to help build a better world."
After his defeat, Mr Massa said: "Obviously the results are not what we expected. I have contacted Javier Milei to congratulate him.
"From tomorrow the responsibility of providing certainty belongs to Milei."
It was a close race to the very end but the former pundit who at one point wielded a chainsaw on the campaign trail, won out.
Javier Milei has been likened to former US President Donald Trump and Brazilian ex-leader Jair Bolsonaro.
He has little political experience but the fact that he is a political outsider was part of his appeal for many voters.
His talk of introducing the US dollar as the country's official currency was met with applause by his supporters. Many economists though say it may spell financial disaster.
In a country where annual inflation is now over 140% and two in five people live in poverty, his win proves that Argentines are fed up with traditional politics and economic disaster.
His victory means they are willing to try something new in an effort to make their lives better.
Reacting to the result, former President Trump posted on his Truth Social platform: "The whole world was watching! I am very proud of you."
White House National Security Adviser Jake Sullivan congratulated Mr Milei on his election "and to the people of Argentina for holding free and fair elections".
Posting on X, he said the US would "look forward to building on our strong bilateral relationship based on our shared commitment to human rights, democratic values, and transparency".
South American leaders have also been commenting on Mr Milei's victory, with Brazil's President Luiz Inácio Lula da Silva posting: "I wish good luck and success to the new government. Argentina is a great country and deserves all our respect."
"Brazil will always be available to work together with our Argentine brothers," he posted on X, formerly Twitter.
On the campaign trail, Mr Milei publicly criticised Lula's policies and branded him an "angry communist".
Colombia's left-wing president, Gustavo Petro, said the result of the election was "sad for Latin America". "Neoliberalism no longer has a proposal for society, it cannot respond to humanity's current problems," he posted on X.
Chile's President Gabriel Boric said he saluted Mr Milei "for his victory and Sergio Massa for his worthy recognition of defeat".
He posted on X: "I wish the Argentine people the best and know that they will always have our respect and support."