Portsmouth claimed their first win at Peterborough since 1961 to leapfrog their opponents in the bottom half of the Championship table.
Erik Huseklepp fired Pompey into an early lead and Jason Pearce's header doubled the advantage before half-time.
Huseklepp grabbed a late third to give Michael Appleton's side their first win in four games and move them up to 16th.
Defeat leaves 17th-placed Peterborough - now without a win in five matches - nine points above the relegation zone.
Portsmouth began with real purpose and after six minutes Marko Futacs's header was palmed away by Joe Lewis. Huseklepp eventually fired home the rebound for his fourth goal of the season.
Peterborough fought back and Pompey keeper Stephen Henderson was relieved to see Paul Taylor's long-range drive fly inches wide of his far post.
Kelvin Etuhu, playing his second match in Portsmouth colours, was causing the home defence problems and almost created a second goal.
He played in Liam Lawrence on the right, but the captain's shot was smartly tipped around the near post by Lewis.
A deserved second goal came five minutes before the break when defender Pearce's run went untracked by the Peterborough defence and he glanced home Lawrence's cross.
The hosts rarely looked like reducing the deficit after the interval, belying their record as the second most prolific scorers in the Championship.
However, Taylor went on a mazy run and sent a shot from the edge of the box drifting wide, before Grant McCann saw a free-kick tipped around the post.
Portsmouth sealed victory thanks to a clinical finish from Huseklepp, who easily turned Ryan Bennett and bent the ball around Lewis and into the far corner.
Peterborough manager Darren Ferguson: "When you concede poor goals at this level you get punished.
"We have always known we could find ourselves in a tough run and we realise that everyone, including some of our fans, will be doubting us.
"But I don't have a problem with that. Myself and the players had a good chat in the dressing room immediately after the game and we are all up for the challenge of turning results around.
"We need to find a way of breaking opposition teams down but we are not going to change the way we play and I'm certainly not going to alter my philosophy.
"Doing things this way got us promoted and had us in the top half of the table earlier in the season."
Portsmouth boss Michael Appleton: "My big gripe with Erik is that he regularly shows flashes of brilliance but lacks the end product.
"We played him in a more advanced role today and he got his rewards with a couple of goals. It is something I expect to see him doing more often."