Jermain pounced on defensive hesitation to latch onto Mido's pass, skipped around goalkeeper Sander Westerveld and slid the ball home from a tight angle to put us 2-0 up at Fratton Park.
Andy Griffin's own goal had earlier handed us a boost just before the break and Jermain's first of the season put the issue beyond doubt.
It secured a first opening day win since the 2000-01 season and Jermain was soon celebrating an England call-up with team-mate Paul Robinson and Michael Carrick.
Head Coach Martin said: "I saw two Portsmouth players running back and I didn't think he was even looking at the goal and then he had just enough pace on the ball.
"But you know that's the difference between a player like him and other players."
Martin was natually pleased with the three points. "We didn't create a lot of chances but if you can come through away games like that with a solid performance defensively and a win, we'll do better this season."
Martin Jol reckons Jermain Defoe's strike at Portsmouth underlined why he's so special as a striker.