The 23 year old was a £300,000 signing for the Millers back in January 2017 but has failed to make a mark with the Millers, only featuring 22 times for the club and spent last season on loan at Afc Wimbledon, then onto Charlton.
Purrington scored for Charlton in their play-off final win which saw them promoted to the Championship.
He has signed a 3 year deal and told Charlton's official website "I can’t wait to get started. There is no better feeling than being back, especially after the unbelievable year we had last year."
The fee is undisclosed.