Another impressive showing from Michael Duff's side sees them leapfrog their opponents in the Sky Bet League One play-off positions.
A Luca Connell scorcher was followed in the second half by Adam Phillips' fifth strike in six games; enough to secure a huge victory on the road for the Reds.
The Reds began with real aggression, leaving no Peterborough player unmarked, constant pressing in order to cause panic in the home backline.
Our hard work paid off when in the 11th minute of the game, a Nicky Cadden cross from the left wasn't adequately cleared and from around 25 yards out, Luca Connell set himself before half-volleying a terrific, powerful effort into the bottom right corner, leaving the home 'keeper no chance.
Sadly, in what was their first real attack of note in the game, Peterborough levelled. Kwame Poku looked ahead of the defence when receiving the ball wide right, but his inch-perfect cross was wonderfully headed into the bottom-left corner, unmarked by Jack Taylor.
Barnsley should have retaken the lead when Jack Aitchison fired towards a goal vacated by Lucas Bergström whose punch was weak, but the Scot's strike was blocked on the line by a Posh defender.
And another wicked Cadden delivery from that left flank evaded everyone, goalkeeper included, but most pertinently, the stretching Devante Cole who was millimetres away from putting the Reds back on top.
The hosts' top scorer, Johnson Clarke-Harris fired an effort just wide of the near post before the break, in what turned out to be quite an even contest by the time that the referee blew his whistle for the interval.
HALF-TIME: Peterborough United 1-1 Barnsley
The second 45 was more clear-cut, as Duff's men shut down the Posh dangermen and when the chance arose, Barnsley took it.
It didn't come without a scare here and there, however. Injuries to Tom Edwards, and then the man who replaced him, Josh Benson saw the back five shuffled on two further occasions, but they held firm on the night.
Just after the hour-mark, the winner arrived. Cadden had been tripped on the left wing, close to the corner flag. The Reds played a neat set-piece routine with the outstanding Herbie Kane feeding a short ball towards the edge of the box for the onrushing Adam Phillips to blast home, through a crowd of bodies.
The graft from both sides continued, but chances for either were short on supply. And after six minutes of added time, the whistle blew, confirming a fourth straight league victory as the Reds moved fourth in the table, just 9 points behind the automatic positions with a game in-hand.
FULL-TIME: Peterborough United 1-2 Barnsley
Collins, Williams, Edwards (Benson (Cundy)), Andersen, Kitching, Cadden, Kane, Connell, Phillips, Aitchison (Norwood), Cole.
Unused: Walton, Larkeche, Martin, Jalo.
