The head of Brazil's National Office for Protection and Civil Defense, Wolnei Wolff, said 99.9% of the blazes in the state had been caused by human action.
He said there had been no lightning strikes or downed high voltage cables which would account for the large amount of blazes.
Four people have been arrested, BBC reported.
Environment Minister Marina Silva called the situation "unusual", saying her team had not seen so many fires ignite in far-flung locations at the same time.
Officials have not said what the motive of those setting the fires may have been, but said the fact they had broken out simultaneously in different parts of the state indicated they had been set on purpose.
Data from the space agency Inpe suggests the number of fires burning in São Paulo in the month of August has been higher than in any August since it started collecting data in 1998.
The agency has registered 5,281 so far this month, compared with 1,104 in the same period last year.
MA/PR
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