"Thank you for believing in me. ... I am here because you have decided," a triumphant Macri said from his campaign headquarters Sunday night as throngs of supporters erupted in cheers.
Macri spoke shortly after Daniel Scioli, Argentine President Cristina Fernandez de Kirchner's hand-picked successor, conceded defeat.
With more than 98% of votes counted, Macri of the Let's Change coalition had won 51.4% of votes, while Scioli had garnered nearly 48.6%, elections officials said
Closely watched vote
For Fernandez, who's slated to leave office in December after eight years at the helm, Sunday's closely watched vote was a test of whether her populist political legacy would endure.
For a region where leftist movements have played a growing role, it's an election that could shift the balance of power.
The election of Macri, a center-right candidate who's mayor of Buenos Aires and the former president of the Boca Juniors football club, could signal a conservative shift for Argentina. Macri has said he wants to rewrite the playbook on Argentina's economy -- a campaign promise that made him popular on Wall Street and drew sharp criticism from his opponents.
In his victory speech Sunday night, Macri promised he'd work to eliminate poverty in Argentina and change the way business is done.
"I also want to say to our Latin American brothers and our brothers around the world, that we want to have good relationships with all countries," he said. "We want to work with everyone. We know that the Argentine people have much to bring to the world, and we hope to find an agenda of cooperation."
Scioli: 'I have given the best in me'
As she tried to convince voters to support Scioli, Fernandez noted her family's legacy, referring to her two terms as President and the four years her husband, the late Nestor Kirchner
, served as Argentina's leader.
"We have never had a period of government of 12 and a half years with this social and economic stability and of constant progress," she said.
And Scioli, who many had been expecting to sail to an election victory during the first round of voting
in October, launched attack ads against Macri in the days leading up to the runoff
and invoked the name of Pope Francis as he encouraged voters to head to the polls Sunday.
Their efforts to drum up a surge in support weren't enough to give Scioli the victory he was seeking.
In a somber speech from his campaign headquarters as votes were still being counted Sunday night, Scioli acknowledged that the margin between him and his opponent was too large to overcome. But he vowed to keep pushing for the ideals Fernandez and her husband espoused.
"I have given the best in me, with passion, as I usually do, convinced that our proposal to the Argentine people was the best option. The people, in this runoff, have chosen differently," he said. "Now I, from wherever it may be, will still defend an ideal, a country project that started 12 years ago."