Former Millers defender Dale Tonge has been assistant to Darren Ferguson at Peterborough, but the 40 year old has come back up north to join up with the Millers again.
Tonge who made over 160 playing appearances for the Millers, has also previously spent time as a coach in a voluntary capacity at Rotherham during the tenure of Paul Warne.
On returning to the Millers, Tonge said: "It’s an exciting time to be a part of the club, especially as it looks to move forward.
"The gaffer is new onto his role, but has fresh eyes and ears and a style of play that the fans are going to enjoy.
"Already having a strong relationship with the backroom staff was a big attraction and their passionate affiliations to the club is clear for everyone to see. That is a big asset to Rotherham."
"The people are willing to go above and beyond for the club. I’ve worked under a lot of successful managers and understand what is needed to win promotions and be successful and I want to bring that experience here to help the gaffer and the lads.”
Andy Warrington will now revert to his specialised position of goalkeeping coach, having previously occupied the role of assistant manager.