Former Portsmouth and Crawley boss Barker appointed assistant manager at Rotherham
He was in charge at Crawley from August 2012 until November 2013 when he was dismissed following seven games without a win or a goal.
Advertisement
Hide AdAdvertisement
Hide AdBarker then had a brief spell in charge of Portsmouth, overseeing 20 games in charge up until another departure in March 2014 before joining forces at MK Dons as number 2 to Karl Robinson.
He made the switch to become first team coach at Charlton in December 2016 to team up with Robinson after his departure from MK Dons.
Barker will now work alongside Paul Warne, who were previously in the same Rotherham side which won promotion to the second tier in 2001.
Advertisement
Hide AdAdvertisement
Hide AdRotherham have just been relegated from the championship and will be taking their place in League 1 next season.
This will be barker's third spell at the Millers, for whom as a player he scored 13 goals in 153 appearances.