BOGOTA, Colombia — Former rebel Gustavo Petro narrowly won a runoff election over a political outsider millionaire Sunday, ushering in a new era of politics for Colombia by becoming the country’s first leftist president.

Mr. Petro, a senator in his third attempt to win the presidency, had 50.5 percent of the votes, while real estate magnate Rodolfo Hernández had 47.3 percent, with almost all ballots counted, according to results released by election authorities.

Mr. Petro will be officially declared winner after a formal count that will take a few days. Historically, the preliminary results have coincided with the final ones.

Mr. Petro’s victory underlined a drastic change in presidential politics for a country that has long marginalized the left.

“Today is a day of celebration for the people. Let them celebrate the first popular victory,” Mr. Petro tweeted. “May so many sufferings be cushioned in the joy that today floods the heart of the Homeland.”

At his headquarters in the capital city of Bogota, a message on a screen read, “Gracias Colombia.” 

The outgoing conservative president, Iván Duque, congratulated Mr. Petro shortly after results were announced, and Mr. Hernández quickly conceded his defeat.

“I accept the result, as it should be, if we want our institutions to be firm,” Mr. Hernández said in a video on social media. “I sincerely hope that this decision is beneficial for everyone.”

Polls ahead of the runoff had indicated Messrs. Petro and Hernández — both former mayors — were in a tight race since they topped four other candidates in the initial May 29 election, though neither got enough votes to win outright and headed into the runoff.

About 39 million people were eligible to vote Sunday, but abstentionism has been above 40 percent in every presidential election since 1990.

The vote came amid widespread discontent over rising inequality, inflation and violence — factors that led voters in the first round to turn their backs on the long-governing centrist and right-leaning politicians and chose two outsiders in Latin America’s third-most populated nation.

Mr. Petro, 62, was once a rebel with the now-defunct M-19 movement and was granted amnesty after being jailed for his involvement with the group.

He has proposed ambitious pension, tax, health and agricultural reforms and changes to how Colombia fights drug cartels and other armed groups. He obtained 40 percent of the votes during last month’s election and Mr. Hernández 28 percent, but the difference quickly narrowed as Mr. Hernández began to attract so-called anti-Petrista voters.

Mr. Petro’s showing was the latest leftist political victory in Latin America fueled by voters’ desire for change. Chile, Peru and Honduras elected leftist presidents in 2021, and in Brazil, the former president, Luiz Inácio Lula da Silva is leading the polls for this year’s presidential election.