Match ends, West Ham United 1, Arsenal 3.
Arsenal came from behind to end their nine-match winless streak as Freddie Ljungberg enjoyed his first victory as interim manager at the expense of his former club West Ham.
Eighteen-year-old Gabriel Martinelli marked his full Premier League debut by side-footing an equaliser which cancelled out Angelo Ogbonna's deflected first-half opener at London Stadium.
Within nine minutes, Nicolas Pepe had curled a magnificent second into the top corner and Pierre-Emerick Aubameyang fired in a third.
The salvo turned the game on its head and piled the pressure on West Ham boss Manuel Pellegrini, whose side have taken four points from their past nine league games and conceded three times in three successive home games.
The Hammers remain a point above the relegation zone in 16th and face a trip to third-bottom Southampton on Saturday. Arsenal move up two places to ninth.
- Pellegrini 'not worried' about relegation after loss to Arsenal
- Ljungberg working on 'game-by-game basis'
- West Ham v Arsenal as it happened, reaction and analysis
- Football Daily: Ljungberg's Arsenal pull three points out of the bag
Arsenal recover from dismal start
Arsenal's victory was all the more remarkable because until Martinelli added to the seven goals he has scored in cup competitions this season, the visitors had been utterly woeful.
Club officials had spoken before kick-off about the improved atmosphere triggered by Ljungberg's appointment as Unai Emery's replacement but it appeared this game would end in frustration, just as the previous two had done under the Swede.
The visitors were bereft of confidence and mild boos from the travelling support accompanied the end of a first half in which their side failed to have a shot on target and went behind when Ogbonna's header bounced in off Ainsley Maitland-Niles.
True, they did not have much luck. Hector Bellerin was injured in the warm-up and when Kieran Tierney was helped off in obvious pain with a shoulder injury sustained in a seemingly innocuous tangle with Michail Antonio, Ljungberg had lost both his first-choice full-backs in the space of half an hour.
Nevertheless, it was pitiful stuff and when Aubameyang surged down the right wing and sent over a cross that flew over everyone and straight out for a throw-in on the other side of the pitch, it was symptomatic of a club apparently heading nowhere fast.
'Nerveless' Martinelli rewards Ljungberg's faith
It was 1977 when Arsenal last went 10 matches without a win. With an away Europa League game against Standard Liege followed by a home encounter with Manchester City to come, at the interval it was not beyond the realms of possibility that the 12-game barren sequence from 1974 was going to be threatened.
With Alexandre Lacazette and David Luiz on the bench, it was two of Arsenal's most inexperienced players who sparked the change in fortune.
Ljungberg had obviously seen enough of Martinelli in two substitute appearances to trust the Brazilian with his first league start. The reward was a nerveless finish when his side needed it most. Sead Kolasinac provided the cross but there was still a lot to do for the Brazilian, who steered a first-time effort into the corner.
Emery paid a club record £72m for Pepe in August. With one league goal all season, the Frenchman has not really lived up to his billing but his goal here, a curling shot into the right-hand corner of David Martin's net, was perfect in its execution.
Aubameyang made certain of a win few would have anticipated 10 minutes earlier when his clinical finish took his tally for the season to 13. It disguised the fact he had been a virtual spectator for the first hour.
At the final whistle, Ljungberg ran to applaud the visiting fans, knowing he had given his own chances of replacing Emery a significant boost.
What now for the unhappy Hammers?
When they beat Chelsea 1-0 nine days ago to end their own winless sequence, it appeared West Ham were on an upward curve.
The combination of boos and thousands of empty seats that accompanied the final whistle on Monday underlined the truth of the matter.
West Ham are perilously close to dropping into the relegation zone, something the club cannot countenance after moving to the 60,000-capacity London Stadium.
Even if Pellegrini survives this defeat, if West Ham lose again at Southampton on Saturday the calls for his dismissal will become piercingly loud.
This was the third home game running in which they had conceded three goals.
The Hammers were not particularly convincing when they were in front. Once they lost the advantage, the lack of confidence so clear in Arsenal's play transferred to theirs.
Record signing Sebastien Haller was left on the bench and even when he was introduced 20 minutes from time, he made no noticeable impact.
Man of the match: Gabriel Martinelli (Arsenal)
'Like a Duracell battery' - what they said
Arsenal interim boss Freddie Ljungberg told BBC Sport: "The players have belief and tried to move the ball with more tempo. West Ham got tired.
"The players ran their socks off and fought. I believe in them. When I could see them put their shift in, I could see the quality. I thought 'it is here for the taking'.
"Martinelli did amazingly. He is like a Duracell battery, he keeps going. Laca [Alexandre Lacazette] is a tremendous player but I had to make a tough decision."
West Ham manager Manuel Pellegrini told BBC Sport: "For 60 minutes there was just one team on the pitch. But football can be like this.
"We made mistakes in moments of defending. The problem was a lack of patience and quality to decide the game with a second goal and we made important mistakes in defence.
"The pressure for me is exactly the same if we win or lose. When you don't have results things are more difficult. If I had not seen the team play the way they did in the first 65 minutes, I might have doubts [about his ability to turn things around].
"After Southampton at the weekend we have a break. We must try to recover as quickly as we can and we must try to win those three points."
Rare Arsenal recovery away from home - the stats
- West Ham have lost three in a row at home in the Premier League for the first time since August 2015.
- Arsenal came from a half-time losing position to win a Premier League away game for the first time since October 2011 (5-3 v Chelsea).
- Gabriel Martinelli is Arsenal's fourth-youngest scorer in the Premier League (18 years 174 days), after Cesc Fabregas, Serge Gnabry and Alex Oxlade-Chamberlain.
- Arsenal's Pierre-Emerick Aubameyang has been involved in 12 goals in his past 11 Premier League London derbies (nine goals, three assists).
- Since his Premier League debut in February 2018, Aubameyang has scored 43 goals in the competition, a joint-high along with Jamie Vardy.
Arsenal conclude their Europa League group phase campaign at Standard Liege on Thursday (17:55 GMT), still needing a draw to be sure of qualification before entertaining Manchester City at Emirates Stadium in the Premier League on Sunday (16:30). West Ham visit Southampton on Saturday (17:30).
- 3CresswellBooked at 49minsSubstituted forMasuakuat 51'minutes
- 11SnodgrassBooked at 24minsSubstituted forHollandat 78'minutes
- 8Felipe AndersonSubstituted forHallerat 70'minutes
- 15C Sánchez
- 3TierneySubstituted forKolasinacat 29'minutes
- 34XhakaSubstituted forGuendouziat 86'minutes
- 19PépéSubstituted forNelsonat 88'minutes
- 23David Luiz
- Mike Dean
- Shots on Target
Second Half ends, West Ham United 1, Arsenal 3.
Attempt blocked. Gabriel Martinelli (Arsenal) right footed shot from the right side of the six yard box is blocked. Assisted by Mesut Özil.
Matteo Guendouzi (Arsenal) wins a free kick in the attacking half.
Foul by Pablo Fornals (West Ham United).
Ainsley Maitland-Niles (Arsenal) wins a free kick on the right wing.
Foul by Michail Antonio (West Ham United).
Corner, Arsenal. Conceded by Ryan Fredericks.
Attempt blocked. Pierre-Emerick Aubameyang (Arsenal) right footed shot from the centre of the box is blocked.
Offside, West Ham United. Pablo Fornals tries a through ball, but Sébastien Haller is caught offside.
Substitution, Arsenal. Reiss Nelson replaces Nicolas Pépé.
Substitution, Arsenal. Matteo Guendouzi replaces Granit Xhaka because of an injury.
Attempt saved. Nathan Holland (West Ham United) left footed shot from outside the box is saved in the bottom right corner. Assisted by Ryan Fredericks.
Ainsley Maitland-Niles (Arsenal) wins a free kick in the defensive half.
Foul by Pablo Fornals (West Ham United).
Substitution, West Ham United. Nathan Holland replaces Robert Snodgrass.
Attempt blocked. Lucas Torreira (Arsenal) right footed shot from the centre of the box is blocked. Assisted by Gabriel Martinelli.
Gabriel Martinelli (Arsenal) wins a free kick on the left wing.
Foul by Ryan Fredericks (West Ham United).
Substitution, West Ham United. Sébastien Haller replaces Felipe Anderson.