The Baggies made up for a 37th minute Chris Brunt penalty miss when Peter Odemwingie put his side in front 13 minutes later when he was found unmarked in the area by Jerome Thomas.
The unlikely lead was doubled when Gunners keeper Manuel Almunia made a blunder two minutes later from a low Gonzalo Jara cross before Thomas made it 3-0 in the 73rd minute when he fired home.
Arsenal rallied with two late goals from Samir Nasri but it wasn't enough as West Brom left North London with a vital three points.
Arsenal: 2 (Samir Nasri 75', 90') West Bromwich Albion: 3 (Peter Odemwingie 50', Gonzalo Jara 52', Jerome Thomas 73') Venue: Emirates Stadium, London Crowd: 60,025 Referee: Michael Oliver Yellow cards:ARS - Manuel Almunia 37', Laurent Koscielny 51', Marouane Chamakh 83', Tomas Rosicky 80' WBA - Jonas Olsson 9', Gonzalo Jara 52', Paul Scharner 68'
ARS Manuel Almunia (gk), Laurent Koscielny (Vela 66'), Sebastien Squillaci, Gael Clichy, Bacary Sagna, Abou Diaby (Rosicky 57'), Alex Song, Marouane Chamakh, Samir Nasri, Andrei Arshavin, Emmanuel Eboue (Wilshere 56')
Subs - Lukasz Fabianski (gk), Tomas Rosicky (57'), Johan Djourou, Denilson, Carlos Vela (66'), Jack Wilshere (56'), Jay Emmanuel-Thomas
WBA Scott Carson (gk), Jonas Olsson, Pablo Ibanez, Nicky Shorey, Gonzalo Jara, Youssuf Mulumbu, Paul Scharner (Reid 71'), James Morrison (Dorrans 78'), Jerome Thomas (Tchoyi 84'), Chris Brunt, Peter Odemwingie