The former Manchester United captain will continue his role as Martin O'Neill's assistant with the Republic of Ireland, despite taking the job at Villa Park.
"Roy is someone who really doesn't need an introduction because he has proved what he is all about," Gardner told the club's official website. "He was a hard-tackling midfielder, a very vocal player and a great person to have on your side.
"As a central midfielder myself, it is amazing to have someone like him here and hopefully he can help as much as possible. To have someone like that to listen to and to take on board their advice is a dream."
Keane will be installed in the coaching setup when Villa begin pre-season training next week.