Barnsley’s play-off push is firmly on after George Williams’ dramatic stoppage-time winner
Williams’ decisive header put the Reds behind sixth-placed Gillingham on goal difference as the Gills escaped Rochdale with a draw.
Chris Forrester was shown red following a fiery altercation with captain Conor Hourihane in the first-half as neither side could break the deadlock on a blustery evening at Oakwell.
Posh were indebted to goalkeeper Ben Alnwick, who produced a string of sensational saves either side of the break, until he was finally beaten late on.
Caretaker chief Paul Heckingbottom named an unchanged line-up to the side that drew 0-0 to promotion-chasing Burton Albion on Saturday.
Meaning that central midfielder Josh Scowen remained at right-back in place of injured youngster James Bree, who was named Apprentice of the Year at the Football League Awards on Sunday.
The Reds almost took the lead instantaneously as Ivan Toney’s header struck the post following Conor Hourihane’s free-kick, only for the linesman to raise his flag for offside.
But it wasn’t plane-sailing for the hosts as Posh began to find their feet after a sluggish opening ten minutes.
The visitors’ first corner of the game saw the recalled Shaquile Coulthirst rise highest, only to glance his header wide of Adam Davies’ goal.
Chris Forrester then unleashed a powerful effort that fizzed wide of the post, which surely would have beaten the sprawling Davies.
The Reds’ faithful grew increasingly frustrated on the half hour mark.
And Heckingbottom’s charges responded as they forced Alnwick into two sensational saves to his right.
Posh were lucky to go in at the break all square, as shot-stopper Alnwick produced a number of fantastic saves.
Hourihane’s deflected shot was tipped round the post for a corner before Marc Roberts was unlucky not to head home his second goal of the campaign.
The game sprung into life after 43 minutes as Forrester saw red following an altercation with Hourihane.
The Irishman reacted angrily to Hourihane appearing to intentionally stamp on his foot.
The pair both received yellows for the initial coming together before referee Michael Bull sent the Posh player off for a second shove.
After the drama subsided Alnwick pulled off an incredible reaction save to tip Winnall’s effort from point-blank range over the bar.
The Reds started the brighter of the two after the break.
Toney’s audacious overhead kick was deflected inches wide for a corner, before Alnwick kept out Alfie Mawson’s curling strike from the resultant corner.
Moments later, Alnwick managed to somehow keep out Josh Brownhill’s ferocious free-kick after Winnall was tripped 25 yards out.
The visitors did however manage a shot on target.
Jon Taylor charged down the left, cut inside and forced Davies into a stunning save to his right.
Alnwick once again kept the Reds’ leading scorer at bay as he stopped Winnall’s powerful volley at the near post on the hour mark.
The Reds’ finally unpicked the lock through George Williams’ maiden Barnsley goal.
The 23-year-old took advantage of a mistake in the visitors’ back-line as he headed home Hammill’s cross from the right to spark scenes of jubilation from the Reds’ faithful and finally get past Alnwick.
Posh chief Graham Westley was sent to the stands moments before the final whistle after an irate outburst at the fourth official and the referee.
Barnsley (4-4-2): Davies 7; Scowen 7, Roberts 6, Mawson 7, G Williams 8; Isgrove 6(Chapman 65, 5), Brownhill 6, Hourihane 7, Hammill 7; Toney 6 (Fletcher 59, 6), Winnall 7.
Subs not used: Townsend, Nyatanga, Khan, McCourt, Tuton.
BOOKED: Hourihane (40)
Peterborough (4-4-2): Alnwick 9, Baldwin 6, Taylor 5, Bostwick 5, Oztumer 4 (Zakuani 45, 5), Maddison 4 (Angol, 45, 5), Almeida Santos 5 (Da Silva Lopes, 90+3 5), Beautyman 5, Fox 5, Forrester 3, Coulthirst 5.
Subs not used: Tyler, Samuelsen, Williams, Nabi.
BOOKED: Forrester (40) Bostwick (45+1), Zakuani (81), Alnwick (88)SENT OFF: Forrester (43)
Referee: Michael Bull (Essex)
MOTM: Ben Alnwick (Peterborough)