The Reds reserve side were beaten by 5 goals to one at the Don Valley Stadium last night despite a rare Steve Hayward goal giving us the lead.
Barnsley started well with Leo Bertos and Robbie Williams going close before the transfer-listed Hayward scored on 13 minutes with a deflected 20-yard shot.
However, the goal just forced the Millers to come out attacking and the deserved equaliser came on 28 minutes through Danny Hudson.
Rotherham's second came on 52 minutes from Hoskins who then turned provider on 65 minutes to create an opening for substitute Liam Kay.
The fourth arrived on 74 minutes as Hoskins pounced after the Reds had failed to deal with a corner and he duly completed his hat-trick with Rotherham’s fifth in the final minutes.
Griff Jones missed a penalty in injury time for the Reds and the result is a huge blow to Barnsley's Avon Insurance League Premier Division title hopes.