The former Manchester United star was the front-runner to become the new boss of Celtic earlier this summer but the move fell through as he elected to remain in his position as Martin O'Neil's No.2 with the Republic of Ireland.
He will now join former Parkhead captain Paul Lambert at Villa Park and combine his club commitments with his role as O'Neil's right-hand man.
"I am delighted to be joining Aston Villa and I look forward to what promises to be an exciting challenge," Keane said.
"I am grateful to both Villa and the Football Association of Ireland for making this happen and I would like also to thank Martin O'Neill.
"I am relishing the prospect of linking up with Paul Lambert and combining both roles to the full advantage of both Villa and Ireland."