AWESOME Albion were in dreamland after pulling off their first win on Arsenal soil in 27 years - and it was fully deserved.
Man-of-the-moment Peter Odemwingie set the Baggies on their way with his fourth goal in five games in the 50th minute.
And, just two minutes later, Gonzalo Jara doubled the visitors' advantage with his first-ever Barclays Premier League goal.
Jerome Thomas stunned the Emirates Stadium even further in the 73rd minute as he added a third against his former employers.
Samir Nasri bagged a late double for the Gunners with strikes in the 75th and 91st minutes to make it an uncomfortable finish for the visitors - but the Baggies held on.
Albion were well worth their first-ever Premier League win at a top-four team and it could have been more had Manuel Almunia not saved Chris Brunt's penalty eight minutes before half-time.
Roberto Di Matteo made two changes - one enforced - as Pablo Ibanez was recalled in place of the injured Gabriel Tamas (calf) to partner Jonas Olsson at the heart of defence.
James Morrison won an instant recall to the starting XI after serving a three-match ban, with Marc-Antoine Fortuné dropping to the bench.
The Baggies boss opted for a more defensive set-up in North London, with Youssouf Mulumbu and Paul Scharner sitting in front of the back-four.
But Albion threatened first in the second minute as Thomas burst down the left and cut the ball just behind Odemwingie in the box.
Mulumbu gifted possession to Gael Clichy in the middle of the park in the ninth minute and the Gunners full-back powered forward before seeing his goalbound drive blocked by Ibanez.
Four minutes later, Nasri pushed forward and fed Andrey Arshavin, who in turn found Emmanuel Eboue on the edge of the area and he swivelled before screwing a shot wide of the left-hand post.
The hosts came within inches of snatching the lead in the 14th minute when Eboue crossed from the right and Arshavin hit the post under pressure from Brunt - and then saw his follow-up deflected onto the same post by Brunt.
Five minutes later, Eboue crossed again and this time Ibanez got a vital touch to prevent Marouane Chamakh heading home from ten yards.
Nasri tried his luck from distance in the 25th minute and saw his effort flash a yard wide of the left-hand post.
But Albion were more than holding their own against the Champions League outfit and, two minutes later, Nicky Shorey played a one-two with Thomas before finding Odemwingie in the box.
The Nigerian cut inside Clichy before seeing Almunia swoop low to his left to finger-tip his shot around the right-hand upright.
Nine minutes before the break, Alex Song played the ball left to Nasri and the France international volleyed high and wide from just inside the box.
Less than 60 seconds later, the Baggies broke and when Brunt slotted Odewingie in behind the Arsenal defence, Almunia scythed him down to concede a clear-cut penalty.
The Gunners keeper was lucky to only receive a yellow card from referee Michael Oliver, giving him the chance to redeem himself by diving low to his right to save Brunt's spot-kick.
It was a real setback for Albion - but you wouldn't have known it.
They continued to attack and, in the 41st minute, Scharner headed over from Brunt's right-wing corner.
Moments later, Sebastien Squillaci missed a glorious chance to take the lead at the other end when he headed Arshavin's right-wing centre over the bar from six yards.
The hosts went close again in the second minute of stoppage time when Bacary Sagna met Arshavin's cross from the same side and forced Scott Carson to tip his header over the bar.
Albion picked up from where they left off after the break.
And five minutes into the second half, they stunned the Emirates Stadium.
Thomas left Sagna for dead on the left-hand byline before pulling the ball back for Odemwingie to slide in and open the scoring.
Just two minutes later, the Baggies were in fantasy land.
Jara passed to Brunt who produced an exquisite back-heel down the right which enabled the Chile star to power on and smash a low 18-yard drive through Almunia's hands and inside the near post.
Wenger responded by making a double change as Jack Wilshere and Tomas Rosicky came on for Eboue and Abou Diaby respectively in the 57th minute.
A minute later, Nasri created space for himself on the edge of the area but saw his powerful goalbound drive blocked by his own team-mate - substitute Wilshere.
Song powered in on Nasri's left-wing centre in the 66th minute - but Ibanez' last-ditch headed block diverted the ball out for a corner.
Wenger went for broke seconds later as striker Carlos Vela came on for centre-half Koscielny.
But Albion appeared to have put the game to bed in the 73rd minute when they added a sensational third.
Brunt managed to beat Almunia to Morrison's through-pass before cutting it back for Thomas to smash home a sweet goal at the club where he started his career - before celebrating wildly in front of the delirious travelling support.
Shell-shocked Arsenal regrouped and, a minute later, Nasri burst to the edge of the box and smashed a shot off the crossbar.
But he was not to be denied and when Sagna fed the ball into his feet in the 75th, Nasri rounded Olsson, who went to ground, and tucked the ball home to reduce the deficit.
Albion had to cope with wave after wave of Arsenal attacks for the rest of the afternoon.
And the final minute provided a scare when Squillaci flicked on a cross, with Carson, who had enjoyed a relatively quiet afternoon, having to dive to his left to hold Chamakh's header.
However, Albion's nerves frayed even further in the first minute of stoppage time when Arshavin found the dangerous Nasri on the edge of the area and he kept his cool to slot home his second of the game.
But the Baggies survived a late volley from substitute Rosicky to cling onto all three points - and record their best-ever Premier League result.
ARSENAL (4-4-1-1): Almunia; Sagna, Koscielny (Vela 66), Squillaci, Clichy; Eboue (Wilshere 57), Song, Diaby (Rosicky 57), Arshavin; Nasri; Chamakh. Subs not used: Fabianski (gk), Denilson, Djourou, Emmanuel-Thomas.
ALBION (4-2-3-1): Carson; Jara, Ibanez, Olsson, Shorey; Mulumbu, Scharner (Reid 71); Brunt, Morrison (Dorrans 78), Thomas (Tchoyi 84); Odemwingie. Subs not used: Myhill (gk), Zuiverloon, Cox, Fortuné.
GOALS: ARSENAL - Nasri (75, 90+1). ALBION - Odemwingie (50), Jara (52), Thomas (73).
BOOKINGS: ARSENAL - Almunia (foul 37), Koscielny (foul 51), Chamakh (foul 64), Rosicky (80). ALBION - Olsson (foul 9), Jara (over-celebrating 52), Scharner (foul 68).
REFEREE: M Oliver (Northumberland).
ALBION STARMAN: Gonzalo Jara.
PIC: WE'VE GUN AND WON IT...Jara and Brunt celebrate Albion's second goal
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