Although financial details were not disclosed, it was the highest profile deal on a quiet final day of trading before the transfer window shut until January.
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Other big moves included Dutch forward Klaas-Jan Huntelaar leaving Milan for Schalke 04 and Premier League Sunderland smashing their transfer record to sign Ghanaian striker Asamoah Gyan from Stade Rennes for a reported fee in excess of 13 million pounds.
The 26-year-old Robinho, who cost City 32.5 million pounds from Real Madrid and has played 80 times for his country, never settled in Manchester and spent six months on loan at Brazilian club Santos last season.
Milan hope Robinho, who has signed a four-year contract, will be a key element in helping them regain the Serie A title they last won in 2004.
"AC Milan annnounce the permanent signing of Robinho from Manchester City. The Brazilian player has signed a four-year contract," Milan said in a statement.
Huntelaar left the San Siro to join Schalke and the German club also recruited Atletico Madrid midfielder Jose Manuel Jurado.
Schalke's Bundesliga rivals VfB Stuttgart signed Italian midfielder Mauro Camoranesi, a 2006 World Cup winner, from Juventus for one year.
KONCHESKY LINK
In England, Fulham full-back Paul Konchesky moved to Premier League rivals Liverpool on a four-year deal where he will link up again with his former manager Roy Hodgson. Liverpool's Argentine defender Emiliano Insua left Anfield on a season-long loan deal for Galatasaray in Turkey.
Another player heading to Turkey was Everton defender Joseph Yobo, poised to joined Fenerbahce on loan before the Turkish transfer window closes on Wednesday.
Arsenal left-back Armand Traore joined Juventus on loan and Croatia goalkeeper Stipe Pletikosa moved from Spartak Moscow to Tottenham Hotspur on a season-long deal.
Stoke City bolstered their squad with two signings, Iceland striker Eidur Gudjohnsen arriving from AS Monaco and Senegal midfielder Salif Diao rejoining the club on a two-year deal.
Birmingham City recruited Spartak Moscow captain Martin Jiranek, Belarus midfielder Aleksandr Hleb and Chile striker Jean Beausejour, while Dutch striker Jan Vennegoor of Hesselink left Championship side Hull City to sign a two-year deal with Rapid Vienna.
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