Jonson Clarke-Harris and Jack Taylor goals wrapped up the a win to give Posh the win they needed, with the Rams losing 1-0 at Sheffield Wednesday.
It means Posh host the Owls in the first leg of the play-off semi on Friday.
Club photographer Joe Dent captured just some of the travelling fans.
Check out this gallery and see if you can spot a familiar face.
All these pictures can be bought by visiting www.theposhpics.com
You can get all the build up to the big match, here.
1. Barnsley 0 Posh 2
Peterborough United fans enjoy the brilliant win at Barnsley as Posh secured a play-off place. Photo: Joe Dent
2. Barnsley 0 Posh 2
Peterborough United fans enjoy the brilliant win at Barnsley as Posh secured a play-off place. Photo: Joe Dent
3. Barnsley 0 Posh 2
Peterborough United fans enjoy the brilliant win at Barnsley as Posh secured a play-off place. Photo: Joe Dent
4. Barnsley 0 Posh 2
Peterborough United fans enjoy the brilliant win at Barnsley as Posh secured a play-off place. Photo: Joe Dent