Goals from Alex Mowatt and Cauley Woodrow gave them an impressive three points which sees them climb above Posh into fifth place.
Mowatt opened the scoring after 25 minutes when he tapped in Dimitri Cavare's fine cross.
Woodrow then produced the standout moment of the game as he doubled the lead with a brilliant 30-yard effort that went into the top corner.
Posh felt the Reds should have been reduced to 10 men when Kenny Dougall appeared to bring down Ivan Toney, in an incident which is sure to dominate boss Steve Evans' post-match thoughts.
Barnsley: Davies, Dougall (Adeboyejo 87), Pinnock, Lindsay, McGeehan, Cavare, Potts (Brown 74), Bahre (Thiam 78), Pinillos, Woodrow, Mowatt.Â
Subs: Greatorex, Hedges, Moncur, Jackson.Â
Peterborough: O'Malley, Naismith (Lyon 69), Daniel (Dembele 56), Woodyard, Tafazolli, O'Hara, Godden (Cummings 57), Ward, Bennett, Toney, Maddison.
Subs:Â Chapman, Reed, Stevens, Cooper.Â Â
Referee: Carl Boyeson
Attendance:Â 12, 843