The 60-year-old helped Saints achieve promotion to the Premier League under Russell Martin.
But he departed following Ivan Juric's appointment as head coach recently.
Calderwood previously served as assistant manager at Northampton, aiding their promotion between 2021 and 2023.
Calderwood said: "I am really pleased to be back. The role here really appeals to me on a number of levels.
"I will be out on the training ground initially and transition more and more towards the technical director role as we move forward and I have been taking my qualifications with that in mind.
"Hopefully I can bring my experience and contacts and the knowledge I have picked up over my career. As everyone will know, I have a lot of affection for the club and I am really pleased to be back."
Manager Kevin Nolan added: "It is the perfect appointment. I can't stress how delighted I am. I worked with Colin at Newcastle, I got on really well with him there he is someone who will support me on and off the field.
"I am really looking forward to having Colin next to me and he forms an excellent addition to a strong technical staff. He will be great for me, great for the other staff and great for the club."