The 26-year-old has penned a four-year contract with the club that has described the move as "the most important signing in our history".
A member of Argentina's 1997 FIFA World Youth Championship winning squad, Aimar was initially dubbed 'the new Diego Maradona'.
The creative midfielder made his senior international debut in 1999 and was a member of the side that disappointed at the 2002 World Cup.
Aimar's early promise saw him move from River Plate to Valencia in a 24 million euros deal in 2001 and he helped Los Che win the Primera Liga title in 2002 and the Primera Liga-UEFA Cup double two years later.
He made three substitute appearances for Argentina at this summer's World Cup.
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A member of Argentina's 1997 FIFA World Youth Championship winning squad, Aimar was initially dubbed 'the new Diego Maradona'.
The creative midfielder made his senior international debut in 1999 and was a member of the side that disappointed at the 2002 World Cup.
Aimar's early promise saw him move from River Plate to Valencia in a 24 million euros deal in 2001 and he helped Los Che win the Primera Liga title in 2002 and the Primera Liga-UEFA Cup double two years later.
He made three substitute appearances for Argentina at this summer's World Cup.
Latest Football News
Valeron woe for Depor
Milan seal Bonera deal
Nurnberg switch for Sibon
Gladbach land Insua
UEFA delay Milan decision