Southampton captain Jason Dodd led from the front to help send Gordon Strachan's men soaring into a Champions League place.
Dodd, Southampton's longest serving player after joining the South Coast club in 1988, was impressive both in defence and on the attack and produced the vital breakthrough with 11 minutes remaining of the first half.
The right-back has been responsible for taking all the Saints' set pieces in the absence of the injured Graeme Le Saux and delivered a magnificent in-swinging corner from the right which eluded everyone to creep in at the far post.
Portsmouth goalkeeper Harald Wapenaar failed to come for the cross and defender Sebastien Schemmel could only head the ball against the crossbar before it nestled in the back of the net.
The result afforded Southampton a third successive Premiership win to lift them up to fourth place in the table and send Pompey crashing to a ninth league defeat in twelve games.
Harry Redknapp's side rarely threatened and Southampton strikers Marian Pahars and James Beattie put the result beyond any doubt in the second half.
Pahars, who has been plagued by ankle injuries over the last year, was on the point of being substituted when he curled home a spectacular second goal on 67 minutes.
The Latvian was allowed time and space to cut in from the left before curling a right-foot shot home from 20 yards to register his first Premiership goal in fifteen months.
Impressive midfielder David Prutton twice went close to extending the advantage in a one-sided derby and it was eventually left to Beattie to net a third in injury time and give the scoreline a more deserving look.
Dodd was once again the creator delivering an inviting cross from the right for Beattie to move in front of his marker Linvoy Primus and plant a close-range header inside the left hand post for his 11th goal of the season.