Conservative Mauricio Macri has been confirmed as the winner in Argentina's presidential elections after his ruling party opponent conceded.
With almost all votes counted, Mr Macri led Daniel Scioli by 51.5% to 48.5%.
He danced on stage at a victory rally at his Buenos Aires headquarters and thanked his staff for their support.
Mr Macri's victory is the first in more than a decade for Argentina's centre-right opposition and ends the 12-year rule of the Peronist Party.
"Today is a historic day," said Mr Macri, addressing thousands of cheering supporters. "It's the changing of an era."
Sunday's victory completes a turnaround for Mr Macri, who is currently mayor of Buenos Aires, after he lost in the first round of voting to Mr Scioli.
But Mr Scioli, who is the governor of Buenos Aires province, did not command enough of a lead to win the vote outright, forcing a run-off - the first in the country's history.
Mr Macri went into Sunday's vote with a comfortable lead in opinion polls, and campaigned on pledges to bring new investment into the ailing economy, tackle crime and fight corruption.
A divided Argentina: Wyre Davies, BBC News, Buenos Aires
As he danced around on the stage, like an embarrassing grandfather at a wedding, Mauricio Macri couldn't care less - he'd produced an election result against a party that has dominated Argentine politics for more than a decade - a result that few would have predicted just a few months ago.
In the end it was close, about a 3% margin over the deflated Daniel Scioli. He'd been handpicked by the outgoing president but could never match Cristina Fernandez de Kirchner's charisma or her bond with the Peronist Party's working class base.
Economic reform will be Mr Macri's number one priority but he gave little away in his victory speech. It will not, though, be easy. Argentina is divided. While most of the white, middle class supporters at the victory rally will hope for a more liberal, open economic climate - the working class, banner waving youth at the ruling party "wake" will hope that warnings about welfare reform and government cuts do not materialise.
The son of one of Argentina's richest men, Mr Macri had a long career in business before entering politics.
In 1991, he was kidnapped and kept captive for 12 days by a gang of corrupt policemen demanding millions in ransom.
Four years later, he became president of Boca Juniors Football Club and used his success at the club as a springboard for his political career.
Mr Scioli, a close ally of outgoing President Cristina Fernandez de Kirchner, had been expected to win by a greater margin in October.
He tried to regain momentum before Sunday's runoff by attacking Mr Macri's market-driven economic policies as a throwback, but failed to regain a lead in the polls.