Match ends, West Ham United 3, Bournemouth 4.
Bournemouth earned their first ever top-flight win as Callum Wilson's hat-trick downed West Ham in a dramatic encounter.
Wilson scored twice in the first half with first-time finishes, with left-back Aaron Cresswell at fault for both.
Mark Noble scored a penalty for West Ham and Cheikhou Kouyate levelled.
Marc Pugh put the Cherries ahead from 16 yards and Wilson scored a penalty, with Carl Jenkinson sent off, before Modibo Maiga ensured a tense finale.
Jenkinson's sending off, for a foul on the lively Max Gradel, was West Ham's fifth red card of the season.
Wilson hat-trick steals the show
Wilson was the undoubted star of the show at Upton Park with his first Premier League goal. And then his second. And then a third.
The former England Under-21 striker scored 23 goals last season following his £3m move from Coventry as the Cherries were promoted to the Premier League.
The 23-year-old, who only made his full Football League debut in 2013, opened the scoring from Simon Francis's cross which he met with a venomous effort. His second was another first-time punt, this time after some poor Cresswell control in the box.
He passed up the opportunity to complete his hat-trick late in the first half when he squared to Gradel but it came from the spot with 11 minutes left.
Defensive errors and poor discipline cost Hammers
West Ham have only themselves to blame for their disappointing start to the season.
A Europa League qualifying exit was followed by a 2-0 win over Arsenal, but back-to-back defeats by some of the Premier League's weaker teams could cause manager Slaven Bilic to worry about his side's mentality.
Cresswell was the club's player of the year in 2014 but was horribly caught out by Francis for Bournemouth's opener and then sloppily controlled the ball which gifted Wilson his second goal.
West Ham improved a bit after the break, leading to their comeback, but the game was beyond them when Gradel destroyed Jenkinson for pace, which led to the defender fouling him for a penalty and red card.
And yet again Bilic - who accepted Jenkinson was rightly sent off - will need to reshuffle his team as a result of suspension, with the Arsenal loanee picking up their fifth red card in nine games this season.
Man of the match: Callum Wilson
'Hammers need better mentality'
West Ham boss Slaven Bilic: "It went wrong from the start. We played a very bad first half. It is very hard to explain three of their four goals - you can't make errors like that at this level. They were more lively and more aggressive but you can't make those mistakes.
"It's not an alarming situation but we can't continue to play without more desire. We simply need a better mentality."
Bournemouth forward Callum Wilson: "It's a fantastic day on a personal note but the important thing is we got the three points.
"In the first two games I think there were nerves, but today I felt we showed real confidence.
"There's a few things we need to brush up on but it's a fantastic result and instils confidence into the team."
Bournemouth manager Eddie Howe: "It was a really strange game. We were in cruise control in the first half but that seemed to go out of the window after half-time when we made some strange decisions.
"The only disappointment at half-time was that we were not more than 2-0 up.
"We have been really heartened by the two previous games but when you don't win then naturally the players will question whether we are doing the right things."
The stats you need to know
- Callum Wilson has scored more away goals than any other player in the top four tiers since the start of last season (15).
- Wilson is only the fifth player in Premier League history to score an away hat-trick for a newly promoted sides (after Phillips, Stewart, Earnshaw and Tchoyi).
- Darren Randolph has conceded 20 goals in his last four league appearances against Bournemouth.
- The Cherries have won 17 penalties since the start of last season; three more than any other club in the top four tiers.
- 12JenkinsonBooked at 79mins
- 21OgbonnaSubstituted forTomkinsat 35'minutes
- 4NolanSubstituted forJarvisat 45'minutes
- 15SakhoSubstituted forMaigaat 73'minutes
- 1BorucBooked at 75mins
- 3S Cook
- 30RitchieSubstituted forA Smithat 90+4'minutes
- 10GradelSubstituted forGoslingat 85'minutes
- 17KingSubstituted forPughat 51'minutes
- 15A Smith
- Jonathan Moss
- Shots on Target
Second Half ends, West Ham United 3, Bournemouth 4.
Corner, West Ham United. Conceded by Eunan O'Kane.
Foul by Adam Smith (Bournemouth).
Aaron Cresswell (West Ham United) wins a free kick on the left wing.
Attempt saved. Dan Gosling (Bournemouth) header from the centre of the box is saved in the top right corner.
Offside, West Ham United. Cheikhou Kouyaté tries a through ball, but Modibo Maiga is caught offside.
Substitution, Bournemouth. Adam Smith replaces Matt Ritchie.
Offside, West Ham United. Darren Randolph tries a through ball, but Modibo Maiga is caught offside.
Attempt blocked. Cheikhou Kouyaté (West Ham United) right footed shot from the right side of the box is blocked. Assisted by Mark Noble.
Corner, Bournemouth. Conceded by Cheikhou Kouyaté.
Offside, West Ham United. Dimitri Payet tries a through ball, but Modibo Maiga is caught offside.
Substitution, Bournemouth. Dan Gosling replaces Max Gradel.
Foul by Callum Wilson (Bournemouth).
James Tomkins (West Ham United) wins a free kick in the defensive half.
Attempt missed. Andrew Surman (Bournemouth) left footed shot from outside the box misses to the right. Assisted by Matt Ritchie.
Goal! West Ham United 3, Bournemouth 4. Modibo Maiga (West Ham United) right footed shot from the centre of the box to the bottom right corner. Assisted by Aaron Cresswell.
Goal! West Ham United 2, Bournemouth 4. Callum Wilson (Bournemouth) converts the penalty with a right footed shot to the bottom right corner.
Carl Jenkinson (West Ham United) is shown the red card.
Penalty Bournemouth. Max Gradel draws a foul in the penalty area.