كتب : وكالات
"The agreement is official: the English champion will be a Rossonero starting from 7 January 2009," Milan said in a statement on their website.
Milan did not reveal when Beckham's loan deal would end.
But the 33-year-old is widely expected to return to Galaxy in March when the new season starts.
Former Manchester United and Real Madrid star Beckham moved to Milan in order to retain his place in England's squad.
Playing for Milan would help Beckham keep up his fitness during the United States close season.
He will join a host of stars at Milan such as Brazilian duo Kaka and Ronaldinho and Ukrainian striker Andriy Shevchenko.