Fabiano's 25th minute goal gave them the victory and an unassailable seven-point lead over second-placed Flamengo with one round of matches remaining.
The Sao Paulo club are considered one of the biggest in Brazil and Sunday's triumph is a record ninth league title, one more than Santos.
But their status has suffered with two relegation seasons since 2002 and often tumultuous internal club politics.
They bounced back after spending 2014 in the second division and, under the stewardship of coach Cuca, they were by far the outstanding team in this year's Serie A, with the best defence and second best attack.
"We're going down in history," veteran utility player Ze Roberto said in a television interview after they lifted the title in front of an elated crowd at the packed Allianz Parque. "This is the start of a new era."
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