The 28-year-old centre forward signed a four-year contract for an undisclosed fee. Sao Paulo will be his ninth club in 10 years.
"We are bringing one of the best strikers on the continent, a starter with Argentina and possibly one of the stars of the next World Cup," Sao Paulo's president Carlos Augusto de Barros e Silva said.
Pratto's defection to a club who acknowledged their need for a regular goalscorer came just hours after their city rivals Palmeiras signed Miguel Borja from Colombia's Atletico Nacional on a five-year deal.
Borja, 24, was one of the outstanding players in a season in which Atletico reached the final of both the Copa Libertadores and the Copa Sudamericana.
Both players are expected to make their debuts in the Sao Paulo state championship that got underway last week.
(Reporting by Andrew Downie Editing by Jeremy Gaunt)
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