The 24-year-old Thorpe, who had captained the Manchester United reserves to the inaugural U-21 Premier League title in 2012-13, has become the red and whites seventh foreigner.
Thorpe will be joining a side coached by former Manchester United forward Teddy Sheringham and which has technical director in Ashley Westwood, also a Manchester United academy graduate.
He has represented the England national team in all age groups from U-16 to U-21, winning 2010 UEFA European Under 17 Championship.
"We welcome Tom to our ATK family. We all are happy to have him in the team. He is an asset which will strengthen our team's defence," ATK principal owner Sanjiv Goenka stated in a release.
"I will learn a lot under Teddy's leadership, and will play to win. But I am sure my stint in ATK will have a remarkable impression on my career path. Above all I would like to thank the management for choosing me to be a part of this squad.