Valencia pounce for Piatti
The 22-year-old, who joined Almeria from Argentina's Estudiantes in 2008, will sign a five-year contract once he has passed a medical, Valencia and Almeria said on their website.
"He is a young lad, with a lot of promise who nonetheless has a great deal of experience in top-level football," Valencia president Manuel Llorente said in a statement.
Piatti comes from the Argentine province of Cordoba, where former Valencia players Mario Kempes, Claudio Lopez and Pablo Aimar were also born.
Valencia finished third in La Liga behind champions Barcelona and Real Madrid for a second consecutive season, securing automatic qualification for the Champions League, Europe's elite club competition.