It was a first win in 10 games for the home side who never looked back after leading scorer Paddy Madden fired them in front in the 40th minute with his 16th goal of the season.
It also ended his own drought of 10 matches without hitting the back of the net when he pounced on a loose ball to sweep home a low left-foot shot from close range.
That gave Scunthorpe the confidence they needed with Dutch striker Kevin van Veen missing a good chance on the stroke of half-time.
It was the home side who dominated much of the second half and they made it 2-0 in the 73rd minute when striker Theo Robinson made it two goals in two games on loan from Doncaster when he touched home a left-wing cross from Gary McSheffrey.
It was a miserable afternoon for Peterborough who lost skipper Jack Payne through illness shortly before the kick-off.
The rest of the team was left looking distinctly off colour as Scunthorpe kept their first clean sheet since mid-October.
Posh’s best two efforts both came from midfielder Michael Bostwick who produced a long-range effort in each half that forced fingertip saves from goalkeeper Luke Daniels.