A live television broadcast on Thursday showed three of the five judges on the Supreme Federal Court voting to convict him.
The judges can still change their decisions until a verdict is pronounced on Friday, however.
In the five-member chamber, a majority of three votes is needed for conviction.
One judge voted for Bolsonaro's acquittal on Wednesday.
US President Donald Trump said hours later that he was "very unhappy" with the conviction. Speaking to reporters as he departed the White House, he said he always found Bolsonaro to be "outstanding."
The conviction, he added, is "very bad for Brazil."
Eduardo Bolsonaro, the former president's son is in the US lobbying for sanctions against Brazil's government over the proceedings.
He has previously claimed credit for pushing the White House to announce 50 per cent tariffs on most Brazilian goods.
The elder Bolsonaro is accused of plotting with allies to stage a coup against the government of his progressive successor Luiz Inácio Lula da Silva following his 2022 election defeat.
According to the prosecution, he had planned to impose a state of emergency and call new elections - but he was unable to win the support of the military leadership.
On 8 January, 2023, supporters of the populist politician stormed Congress, the Supreme Federal Court and the presidential palace in Brasília.
Bolsonaro is also said to have known about plans to assassinate Lula, Vice President Geraldo Alckmin and judge Alexandre de Moraes.
The 70-year-old faces up to 43 years in prison, although the sentence could be lowered to house arrest due to his age.
Bolsonaro has been under house arrest since early August for violating conditions and is being monitored due to the risk of fleeing.
The defence rejected all accusations during the trial and argued that there was no solid evidence of Bolsonaro's involvement in a coup plan.
His lawyers also spoke of a "political trial" in which their client had not had a fair chance.
They criticised de Moraes' role as chief justice, as he played a central role in the investigations and was himself named as a suspected target of the coup plans.
For Bolsonaro's supporters, a "prior judgement" by the court was therefore unavoidable.
In addition to Bolsonaro, former cabinet members and high-ranking military officers are also to be sentenced, including former defence minister Paulo Sérgio Nogueira, former navy chief Almir Garnier and Bolsonaro's former security advisor Augusto Heleno.
They are accused of up to five criminal offences, including attempted coup d'état, participation in an armed criminal organisation and damage to protected monuments.
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