The 31-year-old Zaccardo, who was part of the Italy squad that became world champions in Germany seven years ago, arrives at the San Siro following three and a half seasons with Parma.
Parma receive defender Djamel Mesbah and midfielder Rodney Strasser in exchange, with the latter joining on loan until the end of the current season.
A statement on acmilan.it read: "AC Milan can confirm concluding the following deals with Parma.
"The permanent acquisition of Cristian Zaccardo, the permanent sale of Djamel Mesbah and the temporary loan of Rodney Strasser."
Bizarrely, Zaccardo became the only player to score against Italy in the group stages of the 2006 World Cup, courtesy of his own goal in a 1-1 draw with the USA.
Milan vice-president Adriano Galliani told the club's official website: "We are happy, Zaccardo is a good footballer.
"Is there time for more movement in the transfer window? I'm not sure, we have until the 31st."
Club president Silvio Berlusconi appeared to rule out any deals for Manchester City striker Mario Balotelli or Real Madrid forward Kaka by claiming the Rossoneri cannot afford to "play Monopoly".
Galliani added: "What Berlusconi said... I think he's right. But I'm looking at the numbers and who knows if we can't find a little something."