Keane, who moved to Galaxy from Tottenham Hotspur last year and helped them clinch the MLS Cup championship in November, will stay with Villa until February 25 before returning to the U.S. in time for the new MLS season.
"I'm delighted to be back in the Premier League," Keane told Villa's website. "This is only short-term, of course, but there has been a lot of speculation over the years linking my name to Villa and, finally, I'm here.
"I'm excited about it and I know I'll really enjoy my time here."
The 31-year-old is Ireland's record goal-scorer with 53 and helped his country reach the European Championships finals in Poland and Ukraine later this year.
Keane follows Galaxy team-mate Landon Donovan to the Premier League after he signed on loan for Everton while New York Red Bulls striker Thierry Henry has rejoined former club Arsenal during the MLS off-season.
He could be in line for a Villa debut this coming weekend against Everton, subject to clearance from world governing body FIFA.
"If selected to play on Saturday, I will genuinely be looking forward to it and I'll be excited to be involved again in Premier League action," added Keane, who will link up with former Tottenham team-mate Darren Bent.
"I've been training with the players here for the past few days and it's been great but I'm not naive enough to think I'll just walk straight into the team."
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