D'Alessandro pipped his fellow Argentine Juan Sebastian Veron, who finished second after winning the award in each of the last two seasons with Estudiantes, with 18-year-old Brazilian striker Neymar of Santos in third.
Oscar Tabarez was named the best coach in the annual poll conducted by the Uruguayan newspaper El Pais and recognised by the South American Football Confederation (CSF).
Tabarez led Uruguay to the semi-finals at this year's World Cup in South Africa and finished ahead of Marcelo Bielsa, who took Chile to the second round, and Estudiantes coach Alejandro Sabella, whose side won the Argentine Apertura title
The award has been given every year since 1986 and is based on a poll of around 400 journalists.
Players and coaches have to be based in Latin America for at least part of the year to be eligible.
Previous winners include Brazil's Romario and Cafu and Colombia's Carlos Valderrama.
D'Alessandro, with a fierce left-foot shot, incisive passing and volatile temper, has resurrected his career after joining Internacional in 2008.
Raised at River Plate, the 29-year-old was expected to be one of the outstanding players of his generation.
But, despite winning an Olympic gold with Argentina in 2004, he was seen as never quite having lived up to his potential as he missed out on the World Cup squads for 2006 and 2010.
However, he has been recalled by Argentina since Sergio Batista replaced Diego Maradona as coach following the World Cup.
His club career took him from River Plate to VfL Wolfsburg in Germany, then Portsmouth in England, Real Zaragoza in Spain and back to Argentina to join San Lorenzo.
He joined the growing number of Argentines in Brazil when he went to Porto Alegre-based Internacional in 2008 and helped them win the Libertadores - South America's equivalent of the UEFA Champions League - this year.
