An official count showed Dilma Rousseff’s rival, centrist candidate Aecio Neves, taking just over 48% of the vote.
In her victory speech, Dilma Rousseff said she wanted to be “a much better president than I have been until now”.
She faced protests last year against corruption, record spending on the football World Cup and poor services.
Dilma Rousseff, who has been in power since 2010, is popular with poor Brazilians thanks to her government’s welfare programs.
The vote split Latin America’s biggest country almost evenly in two, along lines of social class and geography.
Dilma Rousseff called on all Brazilians “to unite in favor of Brazil’s future” and said she would seek political reform.
“This president is open to dialogue. This is the top priority of my second mandate,” she told a cheering crowd in the capital, Brasilia.
She also thanked her supporters, especially her political mentor and predecessor Luiz Inacio Lula da Silva.
“I thank from the bottom of my heart our number one militant, President Lula.”
Dilma Rousseff’s re-election for a second term extends the rule of her Workers Party (PT), which came to power in 2002 with President Luiz Inacio Lula da Silva.
Aecio Neves, of the Brazilian Social Democracy Party (PSDB), admitted defeat in a speech to supporters in the southern city of Belo Horizonte.
He thanked the “more than 50 million Brazilians” who voted for him and said he had telephoned the re-elected president.
“I… wished her success in the administration of her next government, and I reaffirmed what I feel should be our greatest priority, to unite Brazil on the basis of an honorable project which dignifies all Brazilians,” he said at the rally.
Aecio Neves was the governor of the southern swing-state of Minas Gerais for eight years.
Both he and Dilma Rousseff had made economic growth and lifting Brazilians out of poverty central to their election campaigns.
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