The Brazilian Socialist Party (PSB) has formally named the internationally-acclaimed environmental campaigner Marina Silvaas its new presidential candidate.
Ms Silva replaces the late Eduardo Campos, who was killed in a plane crash last week.
She was Mr Campos’s running mate and served as environment minister.
She is seen as a leading challenger to President Dilma Rousseff, who’s seeking re-election in the 5 October poll.
“I will give the best I have in me,” said Ms Silva, 56, after the announcement, which was widely expected.
PSB President Roberto Amaral told a news conference she had been chosen unanimously.
Congressman Beto Albuquerque was named the party’s new vice presidential candidate.
A much-admired figure
Ms Silva will test President Rousseff’s status as favourite to win October’s election and make this a much more interesting process than it looked like being barely a week ago, the BBC’s Wyre Davies in Rio de Janeiro says.
In the last presidential election, standing as the Green candidate, Ms Silva polled a credible 20% of the vote and is already a recognisable and much-admired figure across this continent-sized nation, our correspondent adds.
The first test of public opinion after Mr Campos’s death suggested she could surpass the main opposition PSDB candidate Aecio Neves in the first round and beat current President Dilma Rousseff in the second, although both outcomes were within the poll’s margin of error.
But analysts caution that, with the strong emotional reaction to last week’s events, a bounce in the polls was inevitable and the picture could change substantially.
A devout evangelical Christian who overcame poverty, Marina Silva only learnt to read and write when she was 16.
Correspondents say she appeals mostly to young voters who are unhappy with the Brazilian political establishment.
On Sunday, more than 100,000 people in Brazil paid their last respects to the late presidential candidate, Eduardo Campos, a former governor and rising political star.
They attended a funeral Mass and filled the streets of the city of Recife to follow the passage of his coffin.
Mr Campos’s jet crashed on 13 August in bad weather in the port city Santos, near Sao Paulo, killing six other people.
Investigators are still trying to establish the exact causes of the accident.