Mancini swaps Inter for Milan
The former AS Roma player signed for new Inter coach Jose Mourinho in July 2008 but failed to secure a first team place at the Italian champions and had been close to joining Marseille earlier this month.
"AC Milan announce the loan signing of Mancini from Inter, which includes the right to buy half his registration," a Milan statement said on the final day of the January transfer window.
Mancini, who has struggled for any sort of form at Serie A leaders Inter, is likely to act as a deputy for Ronaldinho on the left of second-placed Milan's front three.
A swap deal involving Milan leftback Marek Jankulovski had first been mooted despite the clubs recently trading blows after last month's fractious derby won by Inter, but the Czech looks set to stay put.
Several players including Ronaldo have played for both clubs while midfielders Andrea Pirlo and Clarence Seedorf signed for Milan from Inter.