The French striker, who arrived from Eintracht Frankfurt in a deal worth up to £45m this summer, opened his West Ham account with a tap-in from Felipe Anderson’s cut-back.
And he then acrobatically turned the ball home with an overhead kick from three yards after following up from a corner.
Mark Noble’s third-minute penalty set West Ham on their way to their first win of the season after Manuel Lanzini had been fouled by Abdoulaye Doucoure.
But Watford responded to level with a well-worked Andre Gray goal, with the forward drilling home a right-foot shot from Will Hughes’ pass.
However the Hornets were punished for their wastefulness in front of goal with just three of their 23 shots on target.
Javi Gracia’s side, who sit bottom of the table, have now lost seven successive games in all competitions for the first time since 1999, while West Ham’s victory sees them climb to 13th.
Reversal of fortunes for Watford
At the same stage last term Watford had collected nine points after an impressive start, but a pointless return this time around represents a complete reversal of fortune.
While they did at least register their first goal of the campaign, manager Gracia appears to have problems at both ends of the pitch to contend with.
Despite creating a multitude of opportunities, Gray’s 17th-minute strike was all they had to show for their endeavour.
Gerard Deulofeu’s iNFLuence as a creator was a plus, but he cannot be relied upon to be the prolific forward the club need to improve on last term’s 11th-place finish.
It was a chance, alongside Will Hughes’ extraordinary miss from a yard out after Gray had squared the ball with the score at 1-1, that summed up the hosts’ lack of potency in attack.
At the other end, the presence of Haller, Felipe Anderson and Manuel Lanzini provided too much power, pace and guile for the Watford defence to cope with.
Gracia’s side now face a challenging sequence of fixtures – with a home game against Arsenal sandwiched between trips to Newcastle and reigning champions Manchester City – having shipped 41 goals in their last 18 matches.
Haller takes centre stage
But his first two goals and overall performance suggested that he has the attributes to justify his club record price tag.
The 25-year-old led the line in exemplary fashion and contributed to all three West Ham goals.
His work to chase down possession led to Lanzini winning the penalty that delivered their opener.
The former France Under-21 international then displayed his predatory instincts, holding his run to ensure he was well positioned to capitalise on Anderson’s break down the left.
That allowed him to comfortably claim his first goal for Manuel Pellegrini’s side before acrobatically doubling his tally.
Man of the match – Sebastien Haller (West Ham)
‘It was an amusing game’ – what they said
Pellegrini speaking to BBC Match of the Day: “It was an amusing game of football because both teams play in an attacking way so were always trying to score.
“As a manager, the most important thing is to win but there are different ways to do it. The fans saw a very good game, a good show, and these are the games that bring people to the game.
“Mark [Noble] is a very important player for us, he is the captain of the team. My first idea was not to play him for 90 minutes but he was telling me he was OK.
“[Sebastien Haller’s] goals were two very important goals for him. Coming to the Premier League is not easy. He had just one start in a difficult game against Man City, for a striker the most important thing is to score. The game was open but I told the players it would be before the game. Both teams, especially in the first 45 minutes were very open in both boxes. The impact of [Michail] Antonio was decisive.”
Noble joins esteemed company – the stats
- 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 in the Championship.
- West Ham are undefeated in their last four away matches in the Premier League, winning three of those (D1) – they’d lost seven of their previous eight (D1).
- West Ham are the first club to have had as many as 150 different goal-scorers in the Premier League, following Haller’s first goal for the club.
- Watford had more shots against West Ham (23) than they managed in their first two games of the season combined (19).
- Watford’s Gray has scored 13 Premier League goals since his debut for the Hornets – only Troy Deeney has scored more in this period (14).
- West Ham’s Noble has scored five Premier League goals against Watford – more than he has netted against any other side in the league.
- 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.
Watford host Coventry in the Carabao Cup on Tuesday 27 August (19:45 BST) before they visit Newcastle in the Premier League on Saturday 31 August (15:00 BST).
- 25HolebasBooked at 63mins
- 19HughesSubstituted forSarrat 74′minutes
- 8CleverleySubstituted forQuinaat 86′minutes
- 7DeulofeuSubstituted forWelbeckat 74′minutes
- 21OgbonnaBooked at 76mins
- 7YarmolenkoSubstituted forAntonioat 55′minutes
- 10LanziniSubstituted forC Sánchezat 88′minutes
- 8Felipe AndersonSubstituted forFornalsat 78′minutes
- 15C Sánchez
- Chris Kavanagh
- Shots on Target
Domingos Quina (Watford) wins a free kick in the attacking half.
Foul by Domingos Quina (Watford).
Attempt missed. Etienne Capoue (Watford) right footed shot from outside the box is high and wide to the right following a set piece situation.
Substitution, Watford. Domingos Quina replaces Tom Cleverley.
Attempt missed. Andre Gray (Watford) right footed shot from the right side of the box is too high. Assisted by Tom Cleverley with a headed pass following a corner.
Corner, Watford. Conceded by Issa Diop.
Attempt saved. Abdoulaye Doucouré (Watford) left footed shot from the left side of the box is saved in the centre of the goal. Assisted by Tom Cleverley.
Etienne Capoue (Watford) wins a free kick in the defensive half.
Attempt missed. Tom Cleverley (Watford) left footed shot from the centre of the box is just a bit too high. Assisted by Abdoulaye Doucouré.
Ismaila Sarr (Watford) wins a free kick in the attacking half.
Substitution, Watford. Ismaila Sarr replaces Will Hughes.
Foul by Craig Cathcart (Watford).
Corner, Watford. Conceded by Ryan Fredericks.
Attempt missed. Andre Gray (Watford) header from the centre of the box misses to the left. Assisted by Gerard Deulofeu with a cross following a set piece situation.
Etienne Capoue (Watford) wins a free kick on the right wing.