Barker had fallen out of favour at Hartlepool with boss Danny Wilson now and links up once again with former Rotherham team mate Mark Robins.
Barker told the clubs official website themillers.co.uk: "It is almost four years since I left and I'm delighted to be back.
"There were a few other clubs interested but I still live in Rotherham so the opportunity to come back as a player was too good to turn down. I want to be more successful this time around from a personal and team perspective - I'm going to give it a real go."
We wish Richie all the best in his second stint with the Millers.