Biden and Bolsonaro of Brazil meet for the first time.

President Biden met for the first time on Thursday with President Jair Bolsonaro of Brazil, the leader of the second-largest country in the Western Hemisphere, in a face-to-face discussion that was one of the most anticipated of the ninth Summit of the Americas.

The two leaders were gracious to each other in a photo op before the closed-door session.

“Brazil is a wonderful place with magnificent people,” Mr. Biden said, noting that he had been lucky to visit the “magnificent country” three times in the past. He praised Brazil for making real sacrifices in an effort to protect the Amazon rainforest.

“I think the rest of the world should be able to help you preserve as much as you can” Mr. Biden told Mr. Bolsonaro.

The meeting had the potential to be one of the most tense of the week.

Mr. Bolsonaro is a close ally of former President Donald J. Trump and a supporter of many of the policies that Mr. Biden has tried to combat. He has opened the Amazon to more logging and mining, made it easier to buy guns in Brazil, denigrated the idea of transgender rights and moved Brazil closer to President Vladimir V. Putin of Russia.

But what is most concerning to U.S. officials is Mr. Bolsonaro’s efforts to question the reliability of Brazil’s voting systems ahead of October’s presidential election, a contest in which polls show him trailing by a wide margin. Mr. Bolsonaro has even questioned the legitimacy of Mr. Biden’s electoral victory, mimicking Mr. Trump’s rhetoric, including as recently as this week.

“I will not discuss the sovereignty of another country. But Trump was doing really well,” he said in a local media interview on Tuesday when asked about allegations of voter fraud in the 2020 U.S. elections, which have been repeatedly debunked. “We don’t want that to happen in Brazil.”

Jake Sullivan, Mr. Biden’s national security adviser, told reporters ahead of the meeting between the two leaders that no topic was off limits. “I do anticipate that the president will discuss open, free, fair and transparent democratic elections,” he said.

Sitting next to Mr. Biden on Thursday and speaking in Portuguese, Mr. Bolsonaro addressed the concerns about his country’s democratic traditions.

“I came to office through democracy, and I’m quite certain, when I leave office, it will also be through democratic means,” he said, according to an unofficial translation of his comments.

Several members of Congress have publicly urged Mr. Biden to press Mr. Bolsonaro to increase efforts to find Dom Phillips, a British journalist, and Bruno Pereira, a Brazilian Indigenous expert, who went missing in the Amazon on Sunday after facing threats from illegal fishermen. The Brazilian government’s response has been widely criticized as slow and ineffective.

On Thursday, editors of many of the world’s largest news organizations, including The New York Times, sent a letter to Mr. Bolsonaro, asking him to “urgently step up and fully resource the effort to locate Dom and Bruno.”