The one-time Argentina international, 29, was unable to make an impact in the Premier League, used just six times off the bench after signing from Brazilian side Internacional in January.
Sunderland paid around £4 million for Scocco, who manager Gus Poyet wanted to net the goals to help his side avoid relegation.
Poyet's men avoided the drop, but Scocco played only a minimal part and failed to score.
Scocco started his professional career at Newell's, was also at the club for a loan stint in 2012-13 and has now signed a five-year deal.
A tweet from Newell's read: "First of all, a fan of Newell's...welcome @nachoscocco32!"
Scocco will be presented to fans at the Estadio Marcelo Bielsa on Saturday.
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