Former Manchester United midfielder Keane has previous management experience in charge of Sunderland and Ipswich Town, but will now work as second in command to Lambert.
"Roy brought Sunderland up out of the Championship and into the Premier League, he gained further experience at Ipswich - where we were close rivals while I was manager at Norwich - and he's now working at international level as assistant manager with the Republic of Ireland," Lambert told Villa's website.
"All of that experience and knowledge along with the qualities he has as a longstanding, leading professional in the game means he will bring a lot to the table. We're delighted to have him here."
Keane will start his new role straight away ahead of the Villa squad's return to pre-season training next week.