After a goalless first half, Marc Roberts prodded the Reds ahead nine minutes into the second following a goalmouth scramble, then Adam Hammill’s deflected strike made it 2-0 three minutes later.
Adam Davies enjoyed a relatively stress free afternoon but was called into action to deny Jon Taylor’s low effort on target.
Conor Hourihane and Ryan Kent both forced good saves from Lee Camp before substitute Tom Bradshaw’s looping header and Kent’s 20-yard strike completed the rout four minutes from time.
Victory moves the Reds into third place in the EFL Sky Bet Championship table and means they have not lost a league game at Fortress Oakwell since April 9.
The Reds created their first opening six minutes in when Andy Yiadom’s cross was flicked goalward by the returning Sam Winnall, going just wide of the target.
Marc Roberts came close just past the quarter hour mark, spinning smartly inside the area but scuffing an effort wide, after appeals that Angus MacDonald had been illegally challenged in the area.
Lee Camp was the first goalkeeper called into action midway through the half as Conor Hourihane’s drive span the way of Adam Hammill and his stinging effort was turned away to safety by the Millers stopper.
The visitors’ first meaningful attempt came on 27 minutes as Jon Taylor let fly from the edge of the area but failed to trouble Adam Davies in the Reds goal.
Marley Watkins failed to get on the end of Hourihane’s teasing cross and the ball drifted just wide of the Millers upright and out of play.
Neither side could gather any momentum as the stop-start nature of the half prevented either side from fashioning any clear chances.
Half Time: Barnsley 0-0 Rotherham
The Reds flew out of the traps at the beginning of the second half with Hourihane’s arrowed effort from just outside the area forcing a great save out of Camp.
The breakthrough came nine minutes into the second half as Adam Hammill’s delicious delivery caused pandemonium in the Rotherham defence and Marc Roberts prodded past Camp for his first goal of the season.
Josh Scowen received a caution seconds later for a debatable handball offence on the edge of his own box, which gave Forster-Caskey a chance to shoot from the free-kick but his attempt was straight into the wall. Just moments after that, the Reds had their second.
Hammill drove down the left wing before cutting inside and firing at Lee Camp. His effort took a slight deflection off Kelvin Wilson and wrong-footed the stopper to nestle in the centre of the goal.
Another deflected effort, this time from Watkins, almost looped into Camp’s top corner but the stopper was relieved to watch the ball go wide while Taylor fired a tame shot into the gloves of Davies in reply.
Winnall, who was making his return from a hamstring injury, was withdrawn on 68 minutes and replaced by Stefan Payne, but his introduction was short lived as he hobbled off injured four minutes later, replaced by Tom Bradshaw.
Ryan Kent and Hourihane then tested Camp in quick succession. First, the winger forced the keeper to tip his curling effort away before Hourihane bent the ball into his arms from the edge of the box.
Four minutes from time, Bradshaw put the icing on the cake with his first goal of the season.
Yiadom’s deep cross was met perfectly by the head of Bradshaw, who steered the ball back over the head of Camp to land in the stopper's far corner.
And the 15,293 fans inside Oakwell were rocking again in second half stoppage time as Liverpool loanee Kent charged forward from midfield, before curling a scintillating effort into the top corner of the goal.
The full-time whistle was blown shortly after the fourth goal went in, confirming three points for Paul Heckingbottom's men on a splendid afternoon of football in front of the Oakwell faithful.
Full Time: Barnsley 4-0 Rotherham
Reds: Davies (GK), Yiadom, Roberts, MacDonald, White, Hammill, Scowen, Hourihane (c), Kent, Winnall, Watkins
Subs: Townsend (GK), Moncur, Lee, Evans, Payne, Tuton, Bradshaw
Rotherham: Camp (GK), Kelly (c), Vaulkes, Ward, Forster-Caskey, Taylor, Ball, Wilson, Fisher, Allan, Brown
Subs: Price, Mattock, Wood, Forde, Newell, Smallwood, Yates
Attendance: 15,293 (3,293)