The 33-year-old Beckham is eager to play in Europe during the MLS offseason to remain fit and available for selection by England coach Fabio Capello.
"The English champion will be a Milan player starting on Jan. 7, 2009, the opening day for new signings" a statement on Milan's Web site said.
Milan vice president Adriano Galliani said negotiations were originally slated to start next week, but were moved up. He met with Beckham's lawyers Thursday.
Capello, a former Milan coach, said he approved of the move.
"Milan has made a good acquisition," said Capello, who also coached Beckham at Real Madrid. "In Madrid, I dropped him from the team because he signed with Los Angeles. But he continued to come and train and I put him back on the team."