Elliot Embleton’s goal was the difference in a game which will not last long in the memory at the Stadium of Light, as the visitors drew their fourth league blank in six.
Danny Cowley side’s first-half promise was undone by a lapse at the back from Hayden Carter five minutes before the break.
The visitors created nothing after the break until the late introduction of Michael Jacobs pepped some desperately need life into their attacking play.
The result means it’s one in six for Pompey in the league, with it once again glaringly apparent where their deficiencies lie.
The first half was a tale of Pompey dominating possession and territory - only to be undone by their own mistakes.
The Blues were on the front foot and saw Marcus Harness fire over, after Thorben Hoffman’s ball out hit Tyler Walker.
Pompey were asking the questions but Elliot Embleton forced Gavin Bazunu into a fine save after Ronan Curtis lost possession.
Then it was Sean Raggett caught on the ball by Leon Dajaku but his shot was just past the far post.
Pompey weren’t let off a third time as Hayden Carter lost the ball to Embleton, who exchanged passes with Ross Stewart before firing past Bazunu.
The second half saw the home side assume greater control of the ball and push forward without creating much in tangible openings.
Pompey were looking impotent and incapable of creating an opening of note themselves.
That was until sub Michael Jacobs powered past the red and white shirts and was just crowded out at the depth with 11 minutes remaining.
Jacobs then went it alone after more positive play with his shot at Hoffman from 25 yards prompting chants of ‘we’ve had a shot’ from the away fans.
But it was all too easy for the home side to see out the win through five minutes of stoppage time.