كتب : وكالات
The Rossoneri said on their official site that the 22-year-old was signed as part of a co-ownership deal involving three other players.
Youngsters Nnamdi Oduamadi, Rodney Strasser and Gianmarco Zigoni have moved to Genoa.
Papastathopoulos, who joined Genoa in 2008, was part of Greece's squad in the 2010 World Cup finals in South Africa.