Pompey were 4-1 ahead after only 27 minutes and although Saints improved after the break, the match was already over as a contest.
Redknapp and his assistants Jim Smith and Kevin Bond had to endure a miserable 90 minutes as the team he'd built and then left in November ran Southampton ragged.
Redknapp's Saints now need a miracle to stay in the Premiership while the result should guarantee Portsmouth a third year in the top-flight.
It all went wrong for Redknapp even before the kick off when star striker Peter Crouch pulled a hamstring in the warm-up and had to be replaced at the last minute by Henri Camara.
Four minutes into the match and Portsmouth were awarded a penalty when normally reliable goalkeeper Antti Niemi crashed into Lomana LuaLua. Niemi escaped a card but Yakubu stepped up to convert the spot-kick and Southampton never recovered.
After 17 minutes Patrik Berger took a free-kick on the right and Arjan De Zeeuw rose at the far post to head home a second goal.
Camara scored a superb individual goal three minutes later with a low right-foot shot to give Southampton hope briefly.
But two minutes later another Niemi error, running outside his area to attempt an interception, allowed LuaLua to lift the ball into an empty net from 25 yards.
By now LuaLua was nursing a hamstring injury but he stayed on the pitch long enough to score his team's fourth when he shot home via the inside of a post.
Niemi made two fine saves later to keep the score down and Berger twice went close.
The second half was an anti-climax as Southampton were much tighter in defence, while Jamie Ashdown dived to his right to prevent Nigel Quashie scoring against his old club.
But Portsmouth spurned further opportunities to widen the gap and at the end Redknapp was quickly down the tunnel, his humiliation complete.