The Republic of Ireland international has completed a temporary switch that will run until February 25.
Galaxy will begin their pre-season schedule three days before his return with a match against New England Revolution.
Keane declared his excitement at joining a club with which he had been linked to on more than one occasion in the past.
"I'm delighted to be back in the Premier League," Keane told Aston Villa's official 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. It's only for two months but I will do my absolute best in these two months.
"Villa is a great club, a club with a massive tradition and standing in the game and Villa Park is one of the finest stadiums I've ever played in. 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."
Keane added he expected to have to earn his place in Alex McLeish's starting XI, however.
"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," he said.
"There are really good players here - Darren [Bent], Gabby [Agbonlahor], Emile [Heskey] and young Nathan Delfouneso and Andy Weimann - but if I can help and contribute to the overall effort of the team then hopefully I will be of benefit to Aston Villa. That's my aim."
After spending time on loan at Celtic and West Ham United, Keane left Tottenham permanently in August and lifted the MLS Cup in his debut season in California.
Keane returns to the Premier League sitting just outside of the division's top ten all-time top goalscorers, with 123 goals to his name in the English top flight.
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