"I feel extremely fortunate to have played for some of the best clubs in England and the USA, as well as representing my country at several major tournaments," Keane said in a statement.
"At this stage of my career, the offer to experience a new league and culture in Asia appealed to me.
"To join the current Indian Super League champions is a great pleasure and through hard work I hope to bring the ATK fans more success and enjoyment over the course of the 2017/2018 season."
The 37-year-old will link up with former Tottenham team mate Teddy Sheringham, who was appointed the new ATK manager last month.
Keane scored 126 goals in England's top flight with Coventry City, Leeds United, Wolverhampton Wanderers and Liverpool, in addition to Tottenham. He also netted 68 times in 146 appearances for Ireland before announcing his retirement from international football last year.
(Reporting by Hardik Vyas in Bengaluru; Editing by Andrew Bolton)
