By Josh James at Emirates Stadium
Giroud got the ball rolling in first-half injury time, cracking home a superb left-footed effort across Adrian and in off the post. It was his fifth goal in five appearances against the Hammers.
Arsenal were good value for the lead but they couldn't relax until Aaron Ramsey doubled the advantage on 81 minutes. Giroud was the supplier his time, swapping passes with the Welshman who finished well in front of goal.
Flamini completed the scoring with his first touch after coming off the bench, and it was another well-worked goal. The midfielder slotting home at the far post from Santi Cazorla's low cross.
On another day the Gunners might have had even more, but three goals, and three points was enough to keep Arsenal in third place.
SETTING THE SCENE
With the rest of the top seven not playing until later in the weekend, Arsenal had the opportunity to earn some breathing space in the race for the Champions League places.
Arsène Wenger made five changes to the side that knocked Manchester United out of the FA Cup at the start of the week.
David Ospina, Calum Chambers, Aaron Ramsey, Theo Walcott and Giroud all came into the starting line up, while Wojciech Szczesny, Hector Bellerin, Santi Cazorla and the Old Trafford match-winner Danny Welbeck dropped to the bench. Alex Oxlade-Chamberlain was ruled out with a hamstring injury.
The visitors included former Gunner Alex Song in their midfield, but on loan Carl Jenkinson was unavailable to face his parent club.
Arsenal came into the fixture on the back of a four-game winning streak in the Premier League, and having won each of the past nine meetings with the Hammers.
Emirates Stadium was bathed in sunlight for a rare 3pm Saturday kick-off. With five minutes on the clock Mesut Ozil slipped a pass forward to Giroud inside the box, and with one of his trademark deft flicks, he diverted the ball into Walcott's path.
Walcott – making his first start for over a month – only had Adrian to beat, but he dwelt a split-second too long in possession and James Collins recovered to make the tackle.
The same three Arsenal players were involved a few minutes later as the Gunners launched a counter attack from their own half, which concluded with Alexis Sanchez having a shot charged down inside the box.
Alexis did make the keeper work with his next effort though, forcing Adrian into a good low save with a header from Chambers' inswinging cross.
Arsenal were dominating possession, but on a rare foray forward, West Ham almost took the lead through Kevin Nolan. The midfielder connected sweetly on the edge of the area with a Matt Jarvis cross from the left wing, but Ospina reacted quickly to get his body behind the powerful shot.
The hosts kept moving the ball well and creating chances. The next fell to Ramsey in front of goal, but Chambers' cross arrived at an awkward height and his scuffed effort was pushed over the bar by Adrian.
When West Ham did break forward, Jarvis was invariably at the heart of it, causing problems on the left flank.
But the better chances were being carved out by Arsenal. On the half hour Ramsey measured a pass through to Walcott, who got behind the visiting defence, but his low shot was straight at Adrian.
Then more fine link up play between Walcott, Ozil and Ramsey resulted in the latter standing up a cross for Alexis, whose drive at the far post was parried by the keeper.
Then two minutes before the whistle, Walcott couldn't convert in front of goal after Ozil's shot was parried by Adrian.
But in injury time Arsenal finally broke the deadlock. Ramsey darted into the box, where Giroud nipped the ball away from him and crashing home his stunning strike.
West Ham began the second period brightly. Mark Noble blazed a shot over from close range after Diafra Sakho had collided with Ospina, leaving the Colombian prone on the deck.
Jarvis also flashed a wild shot off target after a corner fell to him 20 yards out.
There was the rare sight of the referee being replaced with an hour gone. Fourth official Anthony Taylor took the place of Chris Foy, who had fallen victim to a calf strain.
Arsenal were continuing to get men forward, but West Ham were an increasing danger on the break. Sakho led one such breakaway, and Arsenal were indebted to Nacho Monreal who just intercepted his pass to the unmarked Jarvis.
The Gunners then had a break of their own, outnumbering the Hammers defence three to two, but Ozil's threaded pass just couldn't reach Giroud who was bearing down on goal.
Arsenal were guilty or overplaying on their next sortie forward. Ozil did brilliantly to find substitute Welbeck on the left wing, and the ball was eventually worked back to the German inside the box. He elected to cross when in front of goal however, and Ramsey's shot went wide.
The second goal had to come, and when it did, it was worth the wait. Giroud and Ramsey swapped passes twice to outfox the West Ham defence, and the Welshman wrapped up the move with a firmly planted left-foot finish.
Another excellent move resulted in the third goal. Flamini couldn't miss at the far post when fellow sub Cazorla's low cross fell to him, after intricate build-up play on the left wing.
Welbeck – who impressed as a second half sub - might have made it four with the last move of the game, but his header was saved low down.
- Ozil 83
- Ramsey Goal!
- Walcott 71
- Giroud Goal!
- Alexis 65 Yellow Card
- Flamini 83 Goal!
- Cazorla 71
- Welbeck 65
West Ham United
- San Miguel del Castillo
- Noble 83
- Jarvis 75
- Sakho Yellow Card
- de Carvalho 83
- Amalfitano 75