But this has been a bitterly fought campaign and the new term - set to start on 5 January - comes amid significant constraints at home and abroad, including animosity from the US president. His win makes him the first in Brazilian history to secure a fourth term, which will last until 2030.
Left-wing leader Lula took 51.79% of votes, becoming the first president since Brazil's return to civilian rule in 1985 to be elected after losing the first round of voting. But Lula's camp gained momentum ahead of election day, focusing on boosting voter turnout after 33 million people did not vote in round one.
Some Bolsonaro fans are pointing out that Zanin was once Lula's lawyer and alleging bias.
The left-wing incumbent president had lost the first round to right-wing challenger Flávio Bolsonaro.
Interest rate futures rose as investors scrutinised Lula's fiscal plans.
World leaders have congratulated Lula on his re-election.
The Sao Paolo stock exchange opened 15% down, sparking a brief suspension before recovering slightly.
Sunday saw the largest drop in abstention between the two rounds of a presidential election since 1989.
His win makes him the first in Brazilian history to secure a fourth term, which will last until 2030.
But Lula's camp gained momentum ahead of election day, focusing on boosting voter turnout after 33 million people did not vote in round one.