The Gunners were shell-shocked when the Baggies went ahead in the third minute through Steven Reid before Peter Odemwingie grabbed a second just prior to the hour-mark.
It was looking like the day the title got away from Arsenal but they managed to fight back and score twice late in the match with Andrei Arshavin supplying a neat finish before Robin van Persie bundled home to give the visitors a share of the points with a 2-2 draw.
Arsenal remained undefeated in12 league games but have only saluted once in their past six outings in all competitions whilst West Brom have not lost in five matches but have a great record against the North London club following their 3-2 win at the Emirates in September.
Arsene Wenger's men have now fallen five points behind league leaders Manchester United, who won 1-0 against Bolton, but thankfully they have a game in hand on the Red Devils.
Sebastien Squillaci came in to replace Johan Djourou in the Arsenal after the Swiss international dislocated his shoulder in the FA Cup loss to Manchester United last weekend whilst captain Cesc Fabregas was sitting out as he tries to get his hamstring right.
It was also the return of Jens Lehmann who was sitting on the Gunners bench after signing an emergency deal following the injury to Wojciech Szczesny.
GOAL West Brom scored immediately when Steven Reid scored his first Premier League goal in five years with a header from a Chris Brunt corner - the Gunners again conceding from a set piece.
The match soon moved to the middle of the pitch with both midfields attempting to get on top but Arsenal were looking a shell of their former selves.
West Brom's James Morrison required treatment for a gash on his head after he and Laurent Koscielny clashed in an aerial duel before Robin van Persie came close to leveling for the visitors when he headed goalward in the 26th minute.
His attempt beat Baggies keeper Scott Carson but it rebounded off the bar only to fall to Aaron Ramsey who volleyed on goal but Carson managed to keep it out.
West Brom seemed much more organised than usual and would have been delighted to go into the half-time break with a one goal lead.
Arsene Wenger brought on Marouane Chamakh for Denilson for the second half as they went about playing their possession game but it didn't work for them as West Brom scored a second goal just before the hour-mark.
GOALArsenal made a huge error between goalkeeper Manuel Almunia and defender Sebastien Squillaci who got in each other's way from a long ball up-field with it falling to Peter Odemwingie who slotted home to make it 2-0.
GOALArsenal finally kicked into gear after conceding a second goal and managed to pull one back in the 71st minute when Andrei Arshavin received a pass from Chamakh before taking a touch and then delivering a superb finish past Carson to give the visitors hope.
GOAL And seven minutes later, Arsene Wenger's men were back in touch thanks to a scrambled goal from van Persie after Nicklas Bendtner headed back across goal for the Dutchman to knock it in from close range after Abdoulaye Meite failed to clear.
Manuel Almunia was called on to make a vital save a few moments later from Marc-Antoine Fortune's shot before the Gunners went about trying to grab a winner but they had to settle for a draw which was salvaged after a horrid start.
West Brom Scott Carson (gk) - Steven Reid, Jonas Olsson, Abdoulaye Meite, Nicky Shorey - Paul Scharner (Cech 46'), Youssuf Mulumbu, James Morrison, Jerome Thomas, Chris Brunt - Peter Odemwingie (Fortune 74')
Subs - Boaz Myhill (gk), Marek Cech (46'), Marc-Antoine Fortune (74'), Gonzalo Jara, Simon Cox, Somen Tchoyi, Gabriel Tamas,
ArsenalManuel Almunia (gk) - Bacary Sagna, Sebastien Squillaci, Laurent Koscielny, Gael Clichy - Samir Nasri, Alex Song, Denilson (Chamakh 46'), Andrei Arshavin - Robin van Persie, Aaron Ramsey (Bendtner 58')