The Republic of Ireland international was on the verge of joining Villa in 1999 from Wolves only for a late hitch over the transfer fee to see him move to Coventry instead.
But the 31-year-old has agreed to move to Villa Park until Major League Soccer restarts in March and will stage a media conference this lunch-time.
Villa received the final paperwork from the Galaxy giving the green light to the move during the early hours of this morning.
Keane could face an instant reunion with his Galaxy team-mate Landon Donovan in Saturday's home Barclays Premier League clash with Everton. Donovan has linked up with the Toffees for a second loan period.
It is also a case of second time lucky for Villa boss Alex McLeish regarding the capture of Keane. McLeish was hopeful of signing Keane 12 months ago when in charge of Birmingham who announced they had agreed a £6 million fee with Tottenham.
But the player instead went on loan to West Ham before leaving White Hart Lane for the United States last summer.
McLeish said: "We have been pursuing Robbie to bring him in with his extra experience in terms of the attack.
"I believe we could be doing with that when you see one or two of our players getting injured over the last couple of weeks. It does leave us a little bit short.
"We've got young kids trying to prove themselves in the Barclays Premier League but it's very scatter-gun the experience they are getting with 10 minutes here, 15 minutes there. I really believe we could be doing with that wee bit of experience that someone like Robbie provides."