West Brom pulled off a shock win with goals from Kanu and Darren Carter.
Philippe Senderos volleyed in a Jose Reyes corner on 16 minutes and a first away win of the Premiership season looked inevitable for the Gunners.
But the Baggies refused to fold, Chris Kirkland repeatedly foiling Arsenal while at the other end Kanu scored.
The Nigerian pounced on a poor Senderos clearance to hammer home an equaliser and the hosts made it two with a wonder strike from substitute Carter.
Carter will take the headlines for his volleyed winner from 20 yards after 76 minutes, but Kirkland deserves the plaudits.
The England squad keeper made telling saves from Reyes, Robert Pires, Dennis Bergkamp, Kolo Toure and Cesc Fabregas to further frustrate Arsenal's stuttering start to the campaign.
Coming into the game the Baggies were mired in their worst run of results under Bryan Robson, but their early energy and effort belied their lowly Premiership status.
However, after the quick opening it looked as if they would fade even faster after Arsenal took a lead their crisp quality deserved.
Bergkamp had the first real sight of goal, but his low, rifled shot was deflected over the bar by a lunging Paul Robinson.
It was a short-lived let-off, Swiss defender Senderos volleying in the resultant corner from Reyes for his first goal in a Gunners shirt.
Kirkland was soon called into action, tipping a Bergkamp effort over, and the flurry of activity in the wrong half of the pitch from the home perspective forced Robson into a change of tactics.
He pushed Horsfield up front alongside Kanu, turning a 4-5-1 into a 4-4-2.
The move worked. Kirkland was still called on too often for comfort, but the hosts drew level after Freddie Ljungberg limped off with a hamstring injury.
Martin Albrechtsen's cross from the right was only half-cleared by Senderos at the near post allowing Kanu to pounce and power home a rising volley on 37 minutes against his former employees.
It was the first goal Arsenal had conceded in 338 minutes of Premiership football and seemed to puncture their self-belief before slowly and surely they reasserted their authority.
But, despite working their way through West Brom's belligerent defensive ranks, there was no getting past Kirkland.
When Robson replaced Kanu with Darren Moore it seemed as if he was happy with a point, but Carter had other ideas, rifling a wonderful shot beyond Jens Lehmann's despairing dive.
It was the first time the Baggies had won in the Premiership having fallen behind and their first home win over Arsenal in 22 years.
West Brom boss Bryan Robson
"I'm delighted for Darren who has been a bit disappointed because I haven't been playing him of late.
"But he got his chance today and responded very well.
"The lads showed great spirit to come back from the position we've been in. People have always been saying we must be down and it's never been like that."
Arsenal manager Arsene Wenger
"We don't speak about the title at the moment, we've other worries.
"It was a strange game. We had plenty of chances and the goals we gave away were without any real danger.
"We played with a good spirit and showed great potential, but we lacked maturity and with a bit more experience we would have won this game easy."
West Brom: Kirkland, Albrechtsen, Curtis Davies, Clement, Robinson, Watson, Wallwork (Carter 68), Greening, Kamara, Horsfield (Ellington 78), Kanu (Moore 70).
Subs Not Used: Earnshaw, Kuszczak.
Goals: Kanu 38, Carter 76.
Arsenal: Lehmann, Lauren, Toure, Senderos, Clichy, Ljungberg (Eboue 36), Fabregas, Flamini (Owusu-Abeyie 77), Pires, Reyes, Bergkamp.
Subs Not Used: Song Billong, Cygan, Almunia.
Booked: Reyes, Clichy.
Goals: Senderos 17.
Ref: B Knight (Kent).