Financial terms of the deal were not disclosed. Keane is due to make $4.5 million in the final year of his contract this season.
Keane joined the team late in the 2011 season and has scored 55 times in 91 MLS appearances. He set career highs for goals (19) and assists (16) last year, winning league MVP honors while leading the team to a second consecutive MLS title.
Still, the signing could be considered something of a gamble by the Galaxy since Keane, who turns 35 next month, has been limited to just five starts and 529 minutes this season because of a groin strain.
"He has been one of the key pieces in our success over the past five seasons," Bruce Arena, the Galaxy's coach and general manager, said in a statement. "We look forward to even more success with Robbie as a member of our team."