The Ireland international, 33, has made 55 league appearances for the Galaxy, scoring 31 goals and helping them to two MLS Cup titles, while also bagging the club's MVP award last season.
And Keane can help the LA club to more silverware over the coming years, after penning his commitment to Bruce Arena's side.
The former Tottenham striker said he was always going to remain with the Galaxy.
"I'm enjoying my football, so that's the most important thing," Keane told the club's website.
"I think it was only going to be a matter of time for me signing. It was done a while ago, and I'm very pleased, and I think it works out well for both myself and the Galaxy."
On their preparation for their MLS opener against Real Salt Lake on Saturday, Keane said everything had been progressing smoothly, including the welcoming of forwards Robert Friend and Samuel to the club.
"It was only pre-season, you can only really judge that when the season starts," he said.
"So far, the pre-season's been good, it's been fine. We look forward to the real business, and that's when you can see how it's come together.
"With the two new strikers that we have, we'll go well because it'll suit me with the way I play."
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