A beaming Roy McFarland saw Chesterfield climb out of the Division Two relegation zone for the first time in five months, with Glynn Hurst and Jamie McMaster doing the second-half damage to inflict Peterborough's seventh home defeat of the season.
The result plunges Barry Fry's strugglers into the bottom four themselves and, really, there was only going to be one winner once Hurst broke the deadlock two minutes into the second half, hooking the ball home from eight yards from a corner.
Hurst went close again before McMaster finished off a sweeping five-man move for Chesterfield's second in the 77th minute. The livewire Leeds loanee was stretchered off near the end, but his injury was not as serious as first thought.
Afterwards McFarland left the talking to his delighted assistant Lee Richardson, who said: "It's good to get a second win in a row and even better to do it with a good performance.
"We coped brilliantly with their pressure and then clinched the points with a really classy goal." Posh had the better first-half openings with Andy Clarke turning on to a Matthew Gill cross to stab wide, and Gill himself was close when meeting Andy Legg's deep centre.
Chesterfield keeper Carl Muggleton had little to do before interval, but proved his worth in the second period with a superb stop to turn Clive Platt's header behind.
Muggleton then kept out a fierce shot from Posh substitute Andy Fotiadis, a save from which Chesterfield raced away to make victory certain with that deserved second goal.
Posh manager Fry commented: "This was a big, big game for us but we never managed to hit the target despite have eight or nine first-half corners.
"It has become evident in the last few weeks that this set of players can't take the pressure of a relegation battle and I may have to look to bring in new faces."