The Black Cats have now not won away since the final day of last season in the Championship and have lost a disappointing ten matches in a row on the road.
Defoe smashed home the penalty after Phil Bardsley brought down Niko Kranjcar in the box to settle a lacklustre affair.
Roy Keane will be disappointed in his side's lack of belief once they went one down and it looks like they could go the whole campaign without winning away from home.
Sunderland started brightly and Kenwyne Jones nearly found an opening from an Andy Reid free-kick but could not direct his header towards goal.
Glen Johnson carved out Portsmouth's first chance in the tenth minute when he beat two defenders and whipped in a beautiful cross that Jermain Defoe just failed to meet.
They went even closer with a superb Sulley Muntari free-kick after Sunderland captain Dean Whitehead had brought down Defoe 20 yards from goal.
Ghana international Muntari bent the ball over the wall, but it just failed to dip under the bar.
Jones gave Sol Campbell a torrid time in the corresponding fixture and was intent on dishing up the same sort of punishment on the former England defender.
The Soca Warrior never gave Campbell a moment's peace and chased every lost cause, but it was Portsmouth that looked the more dangerous side.
Right-back Johnson was booked in the 40th minute for dissent after referee Phil Dowd ruled Danny Collins did not bring him down.
Assistant boss Joe Jordan was sent to the stands for continuing to protest the decision as Portsmouth's frustration started to boil over.
Defoe went close six minutes after the break with the game's first attempt on target. Strike partner Milan Baros got to the touchline and crossed to Defoe who acrobatically hit a volley goalwards. Sunderland goalkeeper Craig Gordon dived to his right to save.
The game was starting to liven up and Grant Leadbitter went close for Sunderland with a scorching 30-yard free-kick which flew just wide of the upright.
Baros, on loan from Lyon, and Defoe linked up again when the former Liverpool striker touched on for Defoe to fire a 20-yard shot straight at Gordon.
Portsmouth took the lead in the 69th minute after Bardsley brought down Kranjcar in the area.
Defoe stepped up to smash the ball into the top left-hand corner after Gordon dived the wrong way as he registered his second goal for Portsmouth since his move from Tottenham.
Sunderland seemed to accept defeat after Pompey had scored and Defoe went close to doubling their lead in the 89th minute but his shot whistled just wide of the right-hand post.