Lula declared his candidacy at a convention of Brazil's Workers' Party, which he helped found 25 years ago.
"I'm here today to tell you that the dream has not ended and that hope has not died," Lula, 60, said on Saturday.
"I am again the candidate not by ambition, but rather because the project to transform Brazil must continue," he added.
Lula said he would keep Jose Alencar, Brazil's vice-president and a conservative businessman, on the party ticket.
Polls suggest that Lula could win the first round of the election in October, comfortably ahead of his closest rival, Geraldo Alckmin, the former Sao Paulo state governor from the centrist Social Democratic Party (PSDB).
If no candidate gets at least 50% of the vote, the top two go to a runoff a few weeks later.
Lula, the country's first left-wing president for more than 40 years, was elected on a platform of aiding Brazil's many impoverished citizens.
In recent months he has been dogged by political scandal, following accusations of corruption involving several prominent politicians and business executives.
However, the scandal does not appear to have affected his ratings.