Westwood served as Bengaluru FC's coach for three years between 2013-16, a stint which led to two I-League titles (13 -14 and 15-16 seasons) and the Federation Cup in 2015.
Westwood joins ISL after coaching Penang FA in the Malaysian Super League.
"I take this opportunity to welcome our technical director Ashley Westwood to the ATK family and believe that it is going to be a fascinating experience for him. Our effort to incorporate professionalism and build a well-knit team has further been heightened by Westwood joining the squad," co- owner of ATK, Sanjiv Goenka said in a statement.