Flavio Bolsonaro said his father had been knifed in the abdomen while in the city of Juiz de Fora in Minas Gerai state.
Brazilian TV showed images of Mr Bolsonaro meeting a crowd of voters before he was stabbed.
The far-right candidate is recovering in hospital following the attack, his son said.
A message posted on his verified Twitter account said the wound was “only superficial”.
Mr Bolsonaro has seen his popularity surge in recent months as Brazil prepares to head to the polls next month.
Recent polling in the country’s unpredictable presidential election puts him as the most likely winner with around 22 percent of the vote.
His nearest rivals are tied with a predicted 12 percent each.
But nearly a third of the electorate is reportedly undecided on which way to vote, with many disillusioned with the widespread corruption which have plagued Brazil.
More to follow…