• Bernardo Arevalo's victory is seen as a repudiation of the political elite in the Central American nation, long the target of corruption allegations.
• "The people of Guatemala have spoken forcefully," Mr Arevalo told reporters. "Enough with so much corruption."
An anti-corruption candidate has swept to a shock win in Guatemala's presidential election.
Bernardo Arevalo's victory is seen as a repudiation of the political elite in the Central American nation, long the target of corruption allegations.
The 64-year old former diplomat led his rival, former first lady Sandra Torres, 58% to 37% with 99% of votes counted.
"The people of Guatemala have spoken forcefully," Mr Arevalo told reporters. "Enough with so much corruption."
Guatemala ranks 150th out of 180 in Transparency International's Corruption Perceptions Index in 2022, tied with Afghanistan and the Central African Republic.
Mr Arevalo's supporters celebrated his victory on the streets of the capital Guatemala City on Sunday night, waving flags and lighting fireworks.
"This triumph represents the defeat of a corrupt system," said one of his supporters, sociologist Jorge Mendoza.
The son of former president Juan José Arévalo, his campaign pulled off a massive upset, defying opinion polls, legal challenges and attempts to disqualify his Movimiento Semilla, or Seed Movement, party.
In the first round of voting last 25 June, Mr Arevalo placed a surprise second to Ms Torres, who has the support of the political elite.
Mr Arevalo will replace unpopular right-wing President Alejandro Giammattei, who is constitutionally barred from re-election. The outgoing leader congratulated Mr Arevalo on X, formerly known as Twitter, and invited him to work on an orderly transition.
Mr Giammattei's rule saw several anti-graft prosecutors arrested or forced into exile. He has also cracked down on critical journalists.
Mr Arevalo will be sworn in on 14 January 2024 as corruption, poverty and gang violence disillusion the nation of 17 million people, pushing thousands to emigrate in search of better lives, many to the US.
"You can no longer live anywhere, because there is so much crime," 66-year-old housewife Maria Rac, an Indigenous Mayan, told AFP news agency.
Mr Arevalo has received congratulatory messages from the ambassadors of the US and the EU and the presidents of neighbouring Mexico and El Salvador.
His victory "means a defeat of the old politics" and marks a new era for Guatemala, independent analyst Miguel Angel Sandoval told AFP.
However, attacks against Arevalo will likely continue, says Risa Grais-Targow, an analyst at political risk consultancy firm Eurasia Group.
"The ruling pact will likely continue to target electoral officials and Arevalo's Semilla party with investigations ahead of January's change in government," she said.
Mr Arevalo will be leading a nation that is in a "very complicated" state, said Ana María Méndez, director for Central America at think tank Washington Office On Latin America.
"I see it more as a transition government, to restore the democratic values that have been broken in Guatemala," she said.