Match ends, Arsenal 0, West Ham United 2.
Arsenal's season started with defeat as West Ham won their first Premier League game under manager Slaven Bilic.
Keeper Petr Cech misjudged Dimitri Payet's free-kick to allow Cheikhou Kouyate to nod in the Hammers' first.
After the break, Mauro Zarate finished from 20 yards after being gifted the ball by the Gunners defence.
Arsenal had 62% of possession, but put just six of 22 shots on target as Aaron Ramsey's first-half effort against the bar was the closest they came.
Same old Arsenal?
Gunners fans, who left the Emirates Stadium in large numbers before the final whistle, will feel like they have seen all this before, not just in the way their side performed, but also in the familiarity of their starting line-up.
Cech's arrival from Chelsea has been Wenger's only summer signing since last term's third-place finish, and it was an uncharacteristic display from the experienced keeper.
The 33-year-old was not only at fault for West Ham's opener, but was wrong-footed by Zarate's strike, although he received little help from his defence as Alex Oxlade-Chamberlain and Francis Coquelin gave the ball away on the edge of their own area.
Olivier Giroud frequently looked isolated up front, which will no doubt increase the hope among Arsenal supporters that a reported deal for Real Madrid forward Karim Benzema can be completed.
Bilic fitting the bill?
Bilic, 46, replaced Sam Allardyce this summer and was handed a remit of getting the east London club playing 'the West Ham way' - a brand of football based around an attacking and entertaining approach.
But the Hammers showed the battling qualities Bilic was renowned for during his spell as a defender with the club from 1996 to 1997.
Reece Oxford, who became his side's youngest-ever Premier League player at the age of 16 years and 237 days, epitomised West Ham's fighting spirit, screening his defence superbly from midfield.
Man of the match - Reece Oxford
Arsenal manager Arsene Wenger: "We were not convincing offensively or defensively. I knew it it could be a tricky game, if you can't win the game, make sure you don't lose it.
"The players were maybe too nervous and put too much pressure on themselves.
"Today we have been hurt mentally and it is a good opportunity to respond."
West Ham manager Slaven Bilic on Reece Oxford: "People were asking: 'Are you sure he is 16?'
"It was going to be a risk but he has got something. I thought we needed someone to sit in front of the defence who is cool and can be a third defender, but who can also play on the ball. I am very proud of him.
"Only on paper he is 16. He is a part of this new generation. Put it this way, if Lionel Messi was in front of me, I would be more fazed than Reece.
"I know we have a good team and I know we have that in ourselves but I wasn't sure how we were going to show it."
The stats you need to know
- West Ham ended a run of nine straight league defeats against Arsenal
- They secured a first win at the Emirates since 2007, a game which was Arsenal's first defeat at their new ground
- Arsenal have failed to score in four of their last five league games at home
- Only Kevin De Bruyne (20) has delivered more assists than Dimitri Payet (18) in the top five European leagues since the start of last season
- All three of Mauro Zarate's Premier League goals for West Ham have come away from home
- Reece Oxford became the second-youngest player to start a Premier League game, after Jose Baxter (for Everton against West Brom in August 2008)
- 2DebuchySubstituted forSánchezat 67'minutes
- 18MonrealBooked at 69mins
- 34CoquelinSubstituted forWalcottat 58'minutes
- 5TomkinsBooked at 37mins
- 21OgbonnaBooked at 7mins
- 35OxfordSubstituted forNolanat 79'minutes
- 16NobleBooked at 5mins
- 10ZárateSubstituted forJarvisat 63'minutes
- 15SakhoSubstituted forMaigaat 90'minutes
- Martin Atkinson
- Shots on Target
Second Half ends, Arsenal 0, West Ham United 2.
Attempt saved. Alexis Sánchez (Arsenal) right footed shot from the left side of the box is saved in the bottom right corner. Assisted by Mesut Özil.
Laurent Koscielny (Arsenal) wins a free kick in the defensive half.
Foul by Modibo Maiga (West Ham United).
Attempt saved. Alexis Sánchez (Arsenal) right footed shot from the centre of the box is saved in the bottom right corner. Assisted by Mesut Özil.
Attempt saved. Modibo Maiga (West Ham United) left footed shot from outside the box is saved in the centre of the goal. Assisted by James Tomkins.
Substitution, West Ham United. Modibo Maiga replaces Diafra Sakho.
Attempt missed. Olivier Giroud (Arsenal) right footed shot from the right side of the six yard box is high and wide to the right. Assisted by Alex Oxlade-Chamberlain with a cross.
Corner, West Ham United. Conceded by Laurent Koscielny.
Corner, West Ham United. Conceded by Laurent Koscielny.
Foul by Mesut Özil (Arsenal).
Mark Noble (West Ham United) wins a free kick in the attacking half.
Attempt saved. Santiago Cazorla (Arsenal) left footed shot from outside the box is saved in the centre of the goal.
Foul by Alexis Sánchez (Arsenal).
Mark Noble (West Ham United) wins a free kick in the defensive half.
Attempt missed. Alex Oxlade-Chamberlain (Arsenal) right footed shot from the right side of the box is close, but misses to the right. Assisted by Aaron Ramsey.
Attempt blocked. Theo Walcott (Arsenal) right footed shot from the centre of the box is blocked.
Attempt saved. Diafra Sakho (West Ham United) right footed shot from the right side of the box is saved in the bottom left corner. Assisted by Cheikhou Kouyaté.
Substitution, West Ham United. Kevin Nolan replaces Reece Oxford.