Peterborough strike duo Jason Lee and Andy Clarke bagged two goals apiece as Cheltenham's seven game unbeaten run disintegrated in the gloom at London Road.
The spirited Posh showing, especially in the opening 45 minutes, also silenced a section of supporters who have been campaigning for the dismissal of manager Barry Fry over the past few weeks.
"Having no game last week was a big help and enabled us to have Andy Clarke back in the side and his play linking-up with Jason Lee made all the difference," said Fry.
"This division has such a narrow line between success and failure it was important for us to get three points in the bag as we move towards the holiday programme. It was also great to hear the crowd getting behind the team and not having a go at me,'" he added.
Veteran Clarke put Posh in front in the 18th minute after cutting in from the left and the 35-year-old had the last word, grabbing a second from close range in the final minute following fine work by David Farrell.
In between Cheltenham's sloppy defensive work was twice punished by Lee. The one time Forest frontman sent in an unstoppable 28th minute header from Jimmy Bullard's out-swinging corner, and then did just as well to glance in another Bullard flag kick after 37 minutes.
Former Posh midfielder Richard Forsyth gave Cheltenham hope with a well-taken goal a minute into the second half, but home keeper Dan Connor put paid to hopes of further progress with two outstanding saves from substitute Damian Spencer.
Cheltenham boss Graham Allner felt this was a turning point, commenting: "A second goal then would have made all the difference and Peterborough would have become anxious in defence. But we just weren't at the races in the first half and had to pay the penalty."