Match ends, Watford 0, Brighton and Hove Albion 3.
New Brighton manager Graham Potter said his side's winning start to the Premier League season was a "dream" as the Seagulls put three past a stunned Watford.
Brighton narrowly avoided relegation last term under Chris Hughton but, with Potter now at the helm, put that misery behind them as Abdoulaye Doucoure's first-half own goal gave them the lead.
The Seagulls had travelled to Vicarage Road on the opening day of last season, losing 2-0 on that occasion, but the scoreline was reversed as substitute Florin Andone, with virtually his first touch of the game, doubled their lead.
Just minutes later, Neal Maupay, who signed for the Seagulls on Monday, justified his £20m price tag with a debut goal, calmly finishing after rounding Watford goalkeeper Ben Foster.
"To score three away from home and not concede is a dream," Potter told BBC Sport.
"The preparation was first class. Overall I thought we tried to do what we set out to do and then you just need a bit of luck and quality.
"We've worked hard every day. There is a really good foundation from what has been done previously - our job is to get better. You can see how they have applied themselves, now we dust ourselves down and don't get carried away."
Watford, who finished 11th in the league last season and reached the FA Cup final, made the more promising start but their late slump at the end of 2018-19, in which they lost their final three matches of the campaign, continued into the new season.
Not since the opening day of the 2015-16 term - when they were in the Championship - had Brighton won their first league match of a season.
"What I know is that they played better than us and deserve the victory today. No excuses. It is the first game and we try to improve for the next one," said Watford boss Javi Gracia.
"The best news is that it is the first game. Sometimes it is good to wake up and improve - we need to do that for the next game.
"There were many individual mistakes. The first and second goal, they took advantage. We expected them to play three at the back and with that shape. We worked in the week to adjust well, but [did so] after they got the win."
New season, new start
Brighton finished 17th in the Premier League last season but a new campaign marks a fresh start for the Seagulls with Potter at the helm.
Managing his first Premier League match since replacing Chris Hughton in May, new signings Maupay, Leandro Trossard, Adam Webster and Aaron Mooy all started on the bench.
After a shaky start, the Seagulls soon settled at Vicarage Road, but little separated the two sides until Watford's Doucoure turned a Pascal Gross cross - destined for the boot of Glenn Murray - into his own net.
It marked the first time the away team had scored - albeit in this case with a helping hand - in a Premier League meeting between Watford and Brighton.
Half-time soon arrived but although it was the hosts who looked the more likely to score after the break, they were undone when Romania international Andone tapped in Davy Propper's cross from close range, moments after being introduced.
Victory was sealed just minutes later as Maupay latched onto a through ball, his goal confirmed after the video assistant referee - making its Premier League debut this season - checked for a possible offside.
Watford chances go begging
Last season's 11th place finish marked a record-breaking campaign for Watford, but manager Javi Gracia chose to postpone the debuts of new signings Ismaila Sarr and Danny Welbeck, who has a long-term ankle injury.
However, his squad was bolstered by the return of Gerard Deulofeu - scorer of 10 goals for Watford last season - after recovering from a muscle problem that plagued his pre-season.
And Deulofeu and captain Troy Deeney both had chances to give their side the lead as the Hornets searched for an opener, only to be left reeling after Doucoure's error.
Watford were unlucky not to level soon after though, when Brighton's new captain Lewis Dunk nearly put the ball into his own net - a Mat Ryan save bouncing awkwardly off his thigh and out for a corner.
After the break, substitute Roberto Pereyra led screams for a penalty after his free-kick from outside the area looked to have struck a hand in the wall, but after a VAR check, they were denied the spot-kick.
That frustration was compounded when Brighton scored their second, then their third, and from then on Watford lost their mojo.
Debutant Craig Dawson, who joined from West Brom in July, went close with a header from a corner that sailed into the arms of Ryan, but Watford's fans were pouring out of Vicarage Road from the moment Brighton's third went in.
'A dream' - what they said
Man of the match - Lewis Dunk (Brighton)
- Watford's 3-0 defeat was their first loss at home against Brighton since September 2012, when they lost 1-0, ending a run of four league games against the Seagulls at Vicarage Road without defeat.
- Brighton secured their biggest victory on the opening day of a league season since 1999-2000 (6-0 v Mansfield in Division 4), whilst Watford suffered their biggest defeat on day one since 2001-02 (3-0 v Manchester City in the second tier).
- Brighton have won their opening game of a top-flight season for just the second time (also in 1980-81 beating Wolves 2-0), drawing two and losing three of their other five games.
- Watford have lost their opening game of a season for the first time since 2006-07 (a 2-1 defeat v Everton), ending a run of 12 matches without defeat on day one (won six, drawn six).
- Brighton have ended a nine-game winless run in the Premier League (D3 L6), their first victory in the competition since beating Crystal Palace back in March.
- Watford have lost three consecutive home league games for the first time since December 2013 in the Championship, when they suffered five consecutive defeats.
- Florin Andone netted Brighton's second goal just 62 seconds after coming on as a substitute.
- Brighton's Neal Maupay is the 13th different French player to score on his Premier League debut; only England (96) has seen more debut goalscorers in the competition.
- Since the start of last season, no side has scored more Premier League own goals than Watford (four - level with Wolves and Burnley).
Watford travel to Merseyside next Saturday to face Everton at Goodison Park (15:00 BST) while Brighton welcome West Ham to the south coast at the same time.
- 7DeulofeuSubstituted forPereyraat 45'minutes
- 18GraySubstituted forSuccessat 75'minutes
- 5DunkBooked at 88mins
- 20MarchSubstituted forBernardoat 90+2'minutes
- 17MurraySubstituted forMaupayat 64'minutes
- 9LocadiaSubstituted forAndoneat 64'minutes
- Craig Pawson
- Shots on Target
Second Half ends, Watford 0, Brighton and Hove Albion 3.
Pascal Groß (Brighton and Hove Albion) wins a free kick in the defensive half.
Foul by José Holebas (Watford).
Substitution, Brighton and Hove Albion. Bernardo replaces Solly March.
Lewis Dunk (Brighton and Hove Albion) wins a free kick in the defensive half.
Foul by José Holebas (Watford).
Attempt blocked. Craig Dawson (Watford) right footed shot from the centre of the box is blocked. Assisted by Troy Deeney.
Attempt blocked. Roberto Pereyra (Watford) left footed shot from outside the box is blocked.
Attempt blocked. Roberto Pereyra (Watford) right footed shot from outside the box is blocked.
Lewis Dunk (Brighton and Hove Albion) is shown the yellow card for a bad foul.
Foul by Lewis Dunk (Brighton and Hove Albion).
Roberto Pereyra (Watford) wins a free kick in the attacking half.
Shane Duffy (Brighton and Hove Albion) wins a free kick in the defensive half.
Foul by Craig Dawson (Watford).
Corner, Watford. Conceded by Dan Burn.
Delay over. They are ready to continue.
Delay in match because of an injury Florin Andone (Brighton and Hove Albion).
Martín Montoya (Brighton and Hove Albion) wins a free kick in the defensive half.
Foul by Etienne Capoue (Watford).