Never before has a candidate who trailed after the first round managed to overturn the deficit and win in the second round.
Bolsonaro's supporters were jubilant at his final campaign rallies, confident that he would bring change to Brazil through his promised tough crackdown on organised crime, as well as tax and public spending cuts.
He campaigned on the eve of polls in Belo Horizonte, the capital of Minas Gerais, which has been a must-win state for any presidential hopeful since the 1980s.
"The time is almost here," Bolsonaro told supporters in a social media video.
"There are only a few hours left before we shape the future of our nation for the next 30 or 40 years."
He has also pledged to release his father, who is currently serving his sentence under house arrest due to ill health.
In Barra de Tijuca, Rio de Janeiro, where Jair Bolsonaro has a home, Bolsonaro supporters rallied in support of him dressed in the national colours.
"Brazil needs change, and Flávio is the person that represents that today, after so many years of the Left destroying the economy and the country," one told BBC News.
Another said that what she most liked about Flávio Bolsonaro was his promise to "end violence in Brazil, end terrorism, narco-trafficking, and to reduce interest rates".