James Beattie produced a double strike to end his and Southampton's goal drought and earn them the bragging rights on the South Coast.
Saints were taking on Portsmouth in the first South Coast derby for nearly eight years for a place in the Carling Cup quarter-finals and Beattie produced the decisive moments of the game with a goal in each half.
Gordon Strachan's side had not scored in more than 400 minutes of football, while Beattie himself had been deprived a goal in his last four games.
But the striker made the most of an inviting cross from midfielder Chris Marsden to net his eighth goal of the season and put his side on their way to victory in the 32nd minute.
Portsmouth defender Boris Zivkovic was dispossessed by Brett Ormerod who fed Marsden to centre for Beattie to finish first-time from six yards out.
The Southampton striker wasn't finished there though, as he put the game beyond doubt in injury time from the penalty spot to end Portsmouth's brave resistance.
Beattie was sent tumbling in the area by Pompey centre-back Arjan De Zeeuw and, after referee Graham Poll had dismissed the Dutchman, the striker picked himself up off the floor to dispatch the spot-kick inside the right hand post.
Swedish defender Michael Svensson had gone close with a header in only the third minute, but it was Portsmouth who had enjoyed the better of the chances before Beattie provided the breakthrough.
Harry Redknapp's team had enjoyed a far from perfect preparation in the build up to the game as they found themselves held up on the motorway after a multi-vehicle accident, which forced kick-off to be delayed by 15 minutes.
But Pompey showed few nerves as they took the game to their more experienced opponents and were unluckly not to establish an early lead.
Nigerian striker Yakubu Ayegbeni twice went close with long-range efforts while veteran striker Teddy Sheringham felt Jason Dodd pulled him back in the penalty area as he prepared to shoot on goal in the 13th minute.
Redknapp's team were restricted to long-range efforts after the interval but almost forced an equaliser before Beattie's late second through wing-back Matthew Taylor.
Patrik Berger slid through a delightful pass for Taylor to close in on, but the ball fell on his wrong foot and goalkeeper Antti Niemi saved comfortably.
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