4:40PM BST 31 Aug 2012
The 27 year-old, who joined the Premier League champions from Hamburg in 2009, underwent a medical this morning before completing the move to the San Siro.
A short statement read: "AC Milan announces that it has bought outright
Manchester
De Jong, who has been given the No 34 shirt, goes straight into the squad for tomorrow's Serie A match at Bologna.
The Holland international made 137 appearances for City during his three and a half years with the club.
De Jong had struggled to find top form with City over the past couple of seasons, which have been blighted by problems sorting out a new contract.
He becomes the second signing of the week for Milan, who also agreed a season-long loan deal for Roma striker Bojan Krkic.
The transfer will go some way towards appeasing Rossoneri fans, who were unhappy to see the club offload the likes of Zlatan Ibrahimovic, Thiago Motta and Antonio Cassano over the summer.
Former Milan striker Filippo Inzaghi, who retired at the end of last season and now coaches the club's youth team, believes De Jong will be a good addition to the squad.
"He'll be very useful, I'm happy he's here," he said.
