"I'm completely focused on ending this season on a high note with my club by winning the MLS Cup," added Beckham, referring to the MLS play-offs.
During his first Milan loan spell, Beckham scored two goals in 18 matches, helping the Serie A side qualify for the Champions League.
He joined Milan on an initial three-month deal in January 2009 that was extended to the end of the season.
Capello reacts to Beckham move after England's win over Belarus (15 October)
"We are very happy to see again David Beckham with the Rossoneri jersey after the splendid experience of last season," AC Milan vice-president Adriano Galliani told the club's official website.
"We are certain that this time in Europe will help the player take part in the next World Cup and to continue after his career in Los Angeles Galaxy, to whom we thank for their time."
Galaxy head coach and general manager Bruce Arena added: "We are pleased to finalise David's loan to AC Milan and we look forward to his return to the Galaxy after the 2010 World Cup."
Beckham's first Serie A game this season could be against Genoa at the San Siro on 6 January.
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