Striker Keane has been lured from Tottenham Hotspur on a two-year contract but the deal was subject to the 31-year-old getting a visa and the Galaxy offloading one of their three designated players.
Under the 'designated player rule', MLS clubs are permitted to sign two players largely outside the league's salary cap and three by paying an extra so-called "luxury tax."
Earlier, the Galaxy said Colombian striker Juan Pablo Angel would be joining Chivas USA in exchange for a third round selection in the 2012 supplemental draft.
The Galaxy already have internationals Landon Donovan and David Beckham on their books and Angel's departure makes way for Keane, holder of 108 caps and scorer of a record 51 goals for his country.
Keane, a prolific scorer for several clubs after starting out at 17 with Wolverhampton Wanderers, will be eligible to make his MLS debut for the Galaxy on Saturday when they host the San Jose Earthquakes.
The Irishman is scheduled to fly to Los Angeles on Thursday and will be introduced to his new team-mates at training on Friday when he will also hold a news conference.
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