Sebastien Haller's first Premier League goals helped West Ham to a 3-1 win over Watford at Vicarage Road.
The Hornets shot themselves in the foot inside two minutes when Abdoulaye Doucoure caught Manuel Lanzini in the box, and Mark Noble stepped up to slot home from the spot on his 350th Premier League appearance.
Watford's response was rapid and they levelled barely 10 minutes later, as Andre Gray was slipped in by Will Hughes and powered low into the far corner.
With the game finely balanced after half-time, Haller netted his first goal in English football from Felipe Anderson's cut-back (64), before an acrobatic second from a corner (73) sealed the Hammers' first win of the campaign.
Watford: Foster (6), Femenia (6), Cathcart (5), Dawson (4), Holebas (5), Hughes (5), Capoue (5), Doucoure (6), Cleverley (5), Deulofeu (7), Gray (7).
Subs: Sarr (5), Welbeck (6), Quina (n/a).
West Ham: Fabianski (6), Fredericks (7), Diop (7), Ogbonna (7), Masuaku (6), Rice (7), Noble (7), Yarmolenko (5), Lanzini (7), Anderson (7), Haller (8).
Subs: Antonio (7), Fornals (6), Sanchez (n/a).
Man of the match: Sebastien Haller
Watford are still searching for their first win since April, and have lost their opening three games of a Premier League season for the first time.
West Ham deliver lesson in finishing
The weight of nerves after two defeats from two was evident immediately with Watford's defence in total disarray as Lanzini cut back inside Ben Foster and went down under Doucoure's challenge inside the box with less than 90 seconds gone.
There were already boos from the stands when Noble slotted home from the spot after a VAR review, and they would have been louder had Lanzini doubled the lead when left unmarked 12 yards out, only to sidefoot a glorious chance wide.
Watford made two enforced changes, bringing in Andre Gray and Tom Cleverley for the injured Troy Deeney and Roberto Pereyra.
Manuel Pellegrini rung the West Ham changes in midfield, making four in total, including a first start since October for Andriy Yarmolenko.
Instead, the frustrated atmosphere turned jubilant when Hughes threaded a fine pass down the outside of Angelo Ogbonna, and Gray let the ball run across him before beating Lukasz Fabianski at his far post.
Both sides struck the woodwork in an end-to-end first period and 10 minutes into the second, Hughes skied a sitter from six yards with the goal gaping.
Watford were made to regret that miss as they were undone by West Ham's quick movement for their second, with Anderson taking over from Haller before racing to the byline and cutting back for his team-mate to tap home.
And moments after Foster had denied Michail Antonio's diving header, the goalkeeper had no chance to keep out West Ham's third. Issa Diop headed against the bar from a corner and Haller leapt to smash home a bicycle kick at the far post.
Watford have now lost their last six Premier League games - matching a run dating back to the end of 2016/17, which ultimately cost then-manager Walter Mazzarri his job.
Javi Gracia: "Today we started well. The first half we competed really well. We conceded an early goal, with a very soft penalty, and it was demanding for us but we reacted well. We dominated the first half, but if you have 23 shots and only three on target, you have to improve.
"They played better in the second half, and the second goal was a tough moment for us. We tried but their counter-attacks were very dangerous for us. We need to find the balance we had attacking and defending. We are conceding many goals, some from difficult situations to accept, but we have to improve and be more clinical.
"Last season we were able to win four games in a row at the start of the season, now we have three defeats. The team is trying to improve, and we'll try to do that in the next game."
Manuel Pellegrini: "We knew it was going to be an open game, with two teams playing attacking football who normally create chances. We both had clear chances before half-time, but when we made the substitution, Antonio changed the game.
"Maybe being open is a different option, we can come and put nine behind the ball, and play long ball, but it's not my way to see that happen, and against Brighton and Watford we tried to win the three points from the start."
- Watford have lost four league matches at home in a row for the first time since a run of five between October and December 2013, when the team played in the Championship.
- West Ham are the first club to have had as many as 150 different goal-scorers in the Premier League, following Sebastian Haller's first goal for the club.
- West Ham's Mark Noble has now scored 25 penalty goals in the Premier League - only Alan Shearer (56), Frank Lampard (43) and Steven Gerrard (32) have scored more in the competition.
Man of the match - Sebastien Haller
Haller could have had a hat-trick with a first-half scissor kick which was easy for Foster to catch, but he started the move which led to the early penalty and got in on the act with two second-half goals.
They might have been from a combined distance of about 12 yards out, but the first owed a lot to excellent movement in between the Watford's two centre-backs, while the second was an excellent technical finish to connect with a bicycle kick.
Firmly off the mark and looking confident, Pellegrini said after the game: "He's a player we've followed a lot in Germany, and when we made the decision to bring him here, we were sure we were bringing in in a very good striker."
Both clubs make their first foray into this season's Carabao Cup in midweek, with Watford hosting Coventry and West Ham travelling to Newport. Both games are on Tuesday night, with the Hammers' tie live on Sky Sports Football and Main Event from 7.30pm; Kick-off 7.45pm.