The 41-year-old – a former Millers team-mate of manager Paul Warne – turned to coaching after ending his playing career.
He gained managerial experience with Bury, Crawley and Portsmouth, and Warne believes he can play an important role as the club look to bounce back at the first attempt following relegation.
“I wanted someone who was an ex-Miller and knows the club, and I like to surround myself with people like that,” Warne told the club’s official website.
“Richie has had good experience in the Championship as a player and as a member of staff, but he also got a team out of League One which will be our goal next season.”
Barker’s most recent position was as a first-team coach at Charlton.