In what was a very entertaining contest on the main pitch here at Oakwell, the Reds emerged victorious despite being reduced to nine men in the second half.
The first 45 saw Barnsley on top, but without creating the sort of clear openings necessary to find themselves in front.
Getting more important minutes as he works his way back to fitness, Conor McCarthy was booked early on after a collision on halfway.
Our first opportunity of the game fell to Trialist who was found well inside the box but his effort from 12 yards was a little too high.
McCarthy found himself on the edge of the box as we approached half time and his vicious strike stung the palms of Posh 'keeper Jake West.
But on the stroke of half time, the visitors took the lead against the run of play as Hisham Chiahr hit a rocket of a shot from 25 yards that found the top corner, giving Reds' stopper Kieren Flavell absolutely no chance.
HALF TIME: Barnsley 0-1 Peterborough United
There were two changes at the break as Charlie Winfield completed his first 45 minutes after returning from injury, replaced by Hayden Pickard while our most recent first team debutant, Mylan Benjamin came on in place of Josiah Dyer.
Within a couple of minutes of the restart, the Reds levelled the contest. A mazy run into the box saw Theo Chapman felled and up stepped Fábio Jaló to fire home the penalty into the bottom left corner.
On the hour-mark, Nathan James was dismissed after he upended a Posh forward breaking through on goal, giving the Reds a real challenge reduced to 10 men.
But despite that setback, the Reds forged ahead as Trialist tucked home a lovely finish from close-range after great build-up play between Jaló and Chapman to find him.
It was 3-1 with 10 minutes remaining fter fine work from Trialist, then Chapman to move the ball into the box allowed substitute Benjamin to blast the ball into the top corner.
Jaló looked to have put the seal on a fine display when he slotted the ball under the 'keeper after a great counter-attack involving Jean Claude Makiessi and the ever-willing Chapman.
But there was still time for Posh to grab what turned out to be a consolation goal as we entered stoppage time, before McCarthy saw a second yellow brandished by the referee after the Irishman had thrown the ball away out by the touchline.
The Reds didn't have to play with 9 men for too long however, and Tom Harban's men recorded an impressive victory.
FULL TIME: Barnsley 4-2 Peterborough United
Flavell, D.Benson (McKay), McCarthy, James, Hickingbottom, Winfield (Pickard), Makiessi, Trialist, Jaló, Chapman, Dyer (Benjamin).
Unused: Ravenhill, Joof.
