The former army officer is currently in second place in the polls behind ex-President Luiz Inacio Lula da Silva.
But Lula is serving a prison sentence for corruption and is unlikely to run.
Mr Bolsonaro has outraged many in Brazil with racist and homophobic comments. But his backers see him as a saviour in a crime-ridden country.
However opinion polls suggest that Mr Bolsonaro would fail to win the election even if he gets into a run-off.
'Brazilian Trump'
Mr Bolsonaro, 63, declared himself a candidate to about 3,000 cheering supporters at a rally in Rio de Janeiro.
The congressman said he would be represented by the Social Liberal Party (PSL) - seen as a lightweight in Brazilian politics.
This means he will have just up to 10 seconds of air time for his TV campaign ads - a serious handicap in the presidential race.
"We don't have a big party. We don't have election funding. We don't have television time," Mr Bolsonaro said.
"But we have what the others don't have, which is you, the Brazilian people."
Mr Bolsonaro is followed by millions of Brazilians on social media, and many refer to him as the "Brazilian Trump".
The candidate, who advocates loosening the country's gun control laws to deal with crime, is also backed by millions of evangelical Christians for his uncompromising anti-abortion stand.
More about: Brazil
