The Datafolha polling institute found that 29% say that Bolsonaro is doing a "great or good" job, down from 33% in July, according to the poll.
Brazilians saying that President Jair Bolsonaro is doing a "bad or terrible" job rose to 38% from 33% previously, following a global outcry over the government's handling of record fires in the Amazon rainforest.
The Datafolha polling institute found that 29% say that Bolsonaro is doing a "great or good" job, down from 33% in July, according to the poll published Monday.
Brazilians who said the Bolsonaro was doing a normal job fell to 30% from 31% previously, within the margin of error.
Datafolha found that 51% of Brazilians thought Bolsonaro had done a bad or terrible job responding to the Amazon fires.
Last month, Brazil's space research agency INPE revealed that the highest number of forest fires were tearing through the Amazon rainforest since 2010, sparking international outcry demanding that the country do more to protect the world's largest tropical rainforest.
Bolsonaro authorized the military to fight the fires after several days of public protests and criticism from world leaders.
The far-right president also engaged in a public war of words with French President Emmanuel Macron, who called the fires "ecocide" and for more to be done to combat them as Group of Seven wealthy nations met in France.
Bolsonaro ultimately said that he would only accept an offer of US$20 million in aid if Macron withdrew "insults" against him.
Datafolha found that 66% of Brazilians said that Brazil should be accepting international aide to combat Amazon deforestation.
Bolsonaro won the presidency last year but has seen his support steadily erode since taking office on Jan. 1.