Four goals after the break gave the Reds an easy victory against a Millers side who offered little in response as the home side dominated.
Shots after 54 minutes and 57 from Marc Roberts and Adam Hammill respectively set Barnsley on their way.
Rotherham's misery was almost complete when substitute Tom Bradshaw - a Millers target in the summer - headed his first Barnsley goal with four minutes to go.
There was still time for Ryan Kent, another player Rotherham had wanted to sign, to fire home a fourth in time added on.
There were boos at the final whistle from Millers fans, and manager Alan Stubbs was the target of abuse from some supporters as he headed for the tunnel.
Stubbs had brought in Jake Forster-Caskey to replace the injured Tom Adeyemi in midfield, consigning Richie Smallwood to the bench.
Former Millers man Aidy White was in the Barnsley line-up and manager Paul Heckingbottom brought Sam Winnall straight back after injury.
Debutant Angus MacDonald replaced centre-half Alfie Mawson, in talks with Swansea City after a Â£5 million bid, for the Reds.
A fairly even first half had brought few clear-cut chances and no sign of the carnage to come.
The Reds were in complete control throughout the second period while the Millers never looked like scoring.
Barnsley (4-4-2): Davies, Yiadom, Roberts, MacDonald, White; Hammill (Lee 87), Scowen, Hourihane, Kent; Watkins, Winnall (Payne 68, Bradshaw 72). Subs not used: Townsend, Moncur, Evans, Tuton.
Rotherham United (4-2-3-1): Camp; Fisher, Ball, Wilson, Kelly; Vaulks, Forster-Caskey (Yates 65); Taylor (Newell 80), Allan, Brown (Forde 83); Ward. Subs not used: Price, Wood, Mattock, Smallwood.
Goals: Roberts 54, Hammill 57, Bradshaw 86, Kent 90+1 (Barnsley).
Referee: Graham Scott (Oxfordshire).