A second-half winner from defender Laurent Koscielny gave Arsenal all three points at West Bromwich Albion, securing third place in the Premier League at the expense of city rivals Tottenham Hotspur.
Arsenal fans were rattled by reports of an early goal at White Hart Lane, but Yossi Benayoun allayed their fears on four minutes, capitalising on a catastrophic error from West Brom keeper Marton Fulop to tap home.
The away side suffered a scare shortly after going in front when Marc-Antoine Fortune fired in a low shot from a tight angle to force a good save from Gunners keeper Wojciech Szczęsny.
West Brom equalised at the 12-minute mark with a controversial goal from Shane Long. The Irish striker was a good two yards offside when he took advantage of sloppy Arsenal defending to level the scores.
The home side took the lead just three minutes later thanks to more poor defending from the visitors. Graham Dorrans got the better of Koscielny to volley home from the edge of the box.
Arsenal could have drawn level on 20 minutes through Robin van Persie, who latched on to a through ball from Tomas Rosicky, but couldn't beat West Brom keeper Jonas Olsson.
There was no denying Arsene Wenger's side on the half hour mark. André Santos's blistering strike proved too much for Fulop to handle, with the home side's keeper at fault again.
Arsenal threw on Theo Walcott as a second half substitute in an attempt to break the deadlock, and the striker looked lively, winning a corner with his first attempt.
The Gunners could have taken the lead when Gervinho lined up for Santos to let fly, but the Brazilian international's shot drifted wide of the far post.
Wenger's men took the lead on 55 minutes when Koscielny bundled home from close range after Fulop completely misjudged a Van Persie corner to add to the West Brom keeper's calamitous afternoon.
The Baggies almost clinched an equaliser on 72 minutes when a great team move led to an opening for Keith Andrews, whose vicious strike was palmed away by Szczesny.
West Brom looked dangerous, but Arsenal's defending was a great deal more solid than in the first period. Alex Song was a particular thorn in the Baggies' side.
The result sees Arsenal edge out Tottenham to third place in the Premier League, booking their place in next season's Champions League.
