The 31-year-old will join David Beckham at the Home Depot Stadium for a fee believed to be around ?1.8million.
Galaxy managed to pull off the deal for Keane, who was linked with a ?3m move to Leicester, before the MLS transfer window closed.
Coming to America: Robbie Keane will link up with David Beckham at LA Galaxy
The club offloaded formerAston Villa striker Pablo Angel so they could add the Ireland strikerto their list of designation players along with Beckham and Landon Donovan.
Keane said it was a 'dream come true' to join the MLS and revealed Beckham had played a role in the move after speaking to him about Galaxy when the former England captain trained with Spurs in January.
'I am delighted, honoured and very excited to be joining the LA Galaxy,' said Keane.
'I have always wanted to come and play in MLS so it's the perfect combination for me and a dream come true. My family and I have already been made to feel very welcome in telephone calls from Tim Leiweke and Bruce Arena.
Advice: Becks told Keane how good life was at Galaxy when he trained with Spurs
'Also when David Beckham came and trained at Spurs recently he couldn't speak highly enough about the Galaxy, their fans and the league?in general, so I can't wait to get over and get started.'
Galaxy boss Bruce Arena said: 'Robbie Keane will be a great addition to our club. I have followed his career and have always respected his ability as a competitor and a goalscorer.
'I believe that he brings qualities in and around the penalty area that we have been lacking. Hopefully, he will be another piece of the puzzle in our quest for the Supporters' Shield and an MLS Cup championship as wellas to advance to the next round of the Champions League.'
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Source: Daily Mail
Source: Daily Mail