The 31-year-old, who has been training with Villa following a number of sessions with former club Tottenham Hotspur, will remain in the Midlands until February 25, subject to FIFA clearance.
It has been well documented that the striker was close to completing a switch to Villa Park, a week after Thierry Henry re-signed for Arsenal from New York Red Bulls on a temporary basis.
"I'm delighted to be back in the Premier League," Keane told the club'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.
"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."
Keane is in contention to make his Villa debut against Everton this weekend.