It was a great day for Posh, who had also celebrated promotion 12 months ago when stepping up from League Two.
Darren Ferguson's men needed to beat Colchester and hope the MK Dons lost at Walsall to ensure their automatic promotion.
That dream ticket arrived thanks to Charlie Lee's 40th minute winner and the Dons' defeat.
Peterborough unsurprisingly dominated this game but home keeper Dean Gerken produced a man-of-the-match display to prevent the visitors from grabbing a killer second goal.
But in the end, Lee's goal was all it took for promotion to be guaranteed.
The left-back scampered onto a precise pass from George Boyd and he exploited acres of space to crash home an unstoppable shot past keeper Gerken.
Peterborough's supporters celebrated in style, but they stepped it up a notch midway through the second half when news filtered through that rivals MK Dons were losing.
Gerken kept them sweating with several good saves, while leading scorer Craig Mackail-Smith was just inches away from converting across from in form winger Boyd.
At the other end, Colchester were nearly party poopers in stoppage time as Mark Yeates lashed in a shot out of the blue that whistled only a foot over the bar.
Seconds later, referee Stuart Attwell blew his final whistle to confirm Peterborough's second successive promotion, with the visiting players celebrating in front of their joyous away fans.
As for Colchester, they had been hampered by dreadful home form throughout the season.
Only seven wins from 24 league and cup games at their new Community Stadium killed off any chance that they may have had of scraping into the play-offs.
U's manager Paul Lambert has promised changes over the summer so you can expect a very different Colchester line-up from the start of next season.