After limping to a 1-1 draw against Plymouth in midweek, Daniel Stendel's men responded in style at London Road as George Moncur and Jacob Brown bagged either side of Brad Potts' double to earn them a notable three points.
Skipper Adam Davies also saved an early penalty as the Reds posted a second 4-0 away victory of the campaign to close to within two points of second-placed Posh.
Davies set the tone for the afternoon when he got down brilliantly to keep out Ivan Toney's 13th-minute penalty, which came after Dimitri Cavare fouled Joe Ward.
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The momentum swung even further in Barnsley's favour a minute later when Peterborough failed to clear and the ball came to Moncur, a Posh loanee last season, to score from close range.
The Reds also made light of an injury to Kieffer Moore as Potts doubled the lead in first-half injury time with a rifled 25-yard effort from a short free-kick.
The former Blackpool midfielder then sent the near-1600 travelling Reds into ecstasy by finding the bottom corner from 20 yards just before the hour.
Substitute Brown put the icing on the cake deep into injury time as he found the same corner to grab his second goal of the season.
Peterborough: Chapman, Naismith, Bennett, Tafazolli, Daniel (Lyon 54), Maddison (Reed 65), Walker (Godden 26), Woodyard, O'Hara, Ward, Toney
Unused subs: Tyler, Dembele, Cooke, Cummings
Barnsley: Davies, Cavare, Pinnock, Lindsay, Williams, Mowatt, Dougall, Bahre (Thiam 89), Potts (Hedges 78), Moncur, Moore (Brown 36)
Unused subs: Walton, McGeehan, Jackson,, Adeboyejo, Brown
Referee: Darren Drysdale
Attendance: 7,683 (1,583 Barnsley fans)