Goals from Nathan Jarman, Griff Jones and Tom Baker gave a young Reds side a convincing victory over the Millers.
Reserves beat the Millers
Barnsley went in front on 20 minutes from a Jarman tap before Jones made it two on 38 after going through on goal.
A Stuart Ludlow strike from a corner gave Rotherham hope before Baker scored for the Reds with five minutes remaining.
Peter Handyside played an hour after his recent injury and should be available for Saturday's match against Plymouth.
Peter Handyside played an hour after his recent injury and should be available for Saturday's match against Plymouth.
BARNSLEY Line-up: Alcock, Harban, Atkinson, Handyside (Batley 58), Laight, Tonge, Heslop, Wroe, Jones, Jarman (Oldham 45), Baker.