"I'm really grateful to the LA Galaxy and AC Milan for making this dream deal happen," the England midfielder told Reuters in an email.
"The perfect result for 2009 would be for me to help Milan to reach the Champions League and for the LA Galaxy to make the playoffs and win the MLS Cup."
A Milan source said his last game for Milan would be May 31 while the contract would run until June 30.
Details on when he will return to the United States and whether he will come back to Milan at the end of the Major League Soccer season in November are yet to be finalised.
"It allows me to play for AC Milan and the LA Galaxy in the same year, and at the same time to continue my commitment to develop soccer in the United States, which is something I am passionate about," Beckham continued.
"My family is happy and settled and will remain in LA."
Beckham joined Milan on loan in January and was due to return to Galaxy on Monday.