A cutely taken goal from Kanu gave Portsmouth their first away win in the Premier League since Boxing Day against a tired Blackburn Rovers at Ewood Park.
Kanu's goal was his 50th in the Premier League
The Nigerian held off Blackburn's Christopher Samba to round Brad Friedel and fire into the net after 25 minutes.
Matt Taylor and Papa Bouba Diop should have extended Portsmouth's lead late on but the win justified Harry Redknapp's decision to bolster midfield.
The defeat ends Blackburn's unbeaten start to the Premier League season.
Rovers looked uninspired after their midweek Uefa Cup trip to Greece but they started far the brighter of the two teams.
And within five minutes they almost took the lead when Roque Santa Cruz hit the post.
The Paraguayan fed David Bentley down the right and continued his run into the box where Bentley found him with a near post cross.
Five minutes later Bentley was again involved when Benni McCarthy half-volleyed high and wide after another enticing cross that Mortem Gamst Pedersen did well to control.
The England midfielder was linking well with Santa Cruz down the right and their roles were reversed a minute later when Bentley fired over.
After riding their luck, Portsmouth began to retain possession and slowly edged their way into the game, but Kanu's 50th Premier League goal came out of the blue.
First Brad Friedel made a fantastic double save from the Nigerian, first denying his header and then a shot while the American lay on the floor.
But Friedel was powerless as Kanu danced around him for the opening goal.
A cross from deep was headed down by Papa Bouba Diop into the space behind the defence and the unmarked Kanu held off Samba before dribbling around Friedel and firing into the net.
If the home fans were looking for a response from Blackburn, they did not get it as Portsmouth calmly held on to the ball and used their packed midfield to its best effect.
Mark Hughes was a picture of frustration on the touchline and it wasn't long before the players were also losing their rag with each other too.
Following the break, and with more pressure failing to make any breakthrough, Hughes made a double substitution with Tugay and Matt Derbyshire coming on for David Dunn and the ineffective McCarthy.
But without a driving force, it still appeared as if the Rovers' players were hoping for a turnaround rather than doing anything to force the issue.
And in Sol Campbell and Sylvan Distin, they were coming up against a resolute backline which was amply protected Diop, Sean Davis and Sulley Muntari.
Indeed, if anyone looked like scoring it was Pompey when Davis made the wrong decision and elected to cross rather than shoot after 54 minutes.
But he was not the only guilty one as first Diop skewed his shot wide from six yards after a Benjani cross and then Taylor shot straight at Friedel.
Blackburn were limited to few chances and a Pedersen header and a Bentley free-kick was close as they came.
Blackburn manager Mark Hughes:
"We don't want to use the midweek defeat as an excuse. We aspire to be a club who plays in Europe and can come back and win games at Premier League level.
"We were lacking in the intensity we normally play with and when you lack that we do not have the influence on the game we usually have.
"They defended very well, their back four was rock solid and it was very difficult for us to draw them out into areas they didn't want to go. If we capitalised on the first 20 minutes it might have been a different game."
Portsmouth coach Tony Adams:
"It was an outstanding performance. Resilient. Sol Campbell makes a difference to us and we've done a lot of work in training this week, espeically with the front guys.
"Kanu is one of those players who can open doors, hold up the ball and bring others into play. There are not many around and he's a special player."
Blackburn: Friedel, Emerton, Samba, Ooijer, Warnock, Bentley, Savage, Dunn (Kerimoglu 56), Pedersen (Roberts 79), Santa Cruz, McCarthy (Derbyshire 56).
Subs Not Used: Brown, Mokoena.
Portsmouth: James, Johnson, Campbell, Distin, Hreidarsson, Muntari, Davis, Diop, Kranjcar (Taylor 86), Kanu (Mwaruwari 69), Utaka (Pamarot 90).
Subs Not Used: Ashdown, Nugent.
Booked: Diop, Muntari, James.
Goals: Kanu 25.
Ref: Chris Foy (Merseyside).