Long-range strikes from Abdoulaye Doucoure and Daryl Janmaat ended Southampton's unbeaten start and lifted Watford into the top four.
Goals either side of half-time secured a 2-0 victory for the upwardly-mobile Hornets as Saints' struggles in front of goal continued.
Doucoure pounced shortly before the break from outside the box and Janmaat hit a second-half cracker just four minutes after coming on as a substitute.
TWEET OF THE MATCH
"Have Watford been practising long-range shots? Daryl Janmaat and Abdoulaye Doucoure might have been... #SOUWAT" - the Premier League's official account, with pictures of the two strikes (https://twitter.com/premierleague/status/906542249923751936).
STAR MAN: Richarlison.
Watford's Brazilian youngster looks a real find, a constant threat down the left with pace to burn and an eye for a trick or two.
MOMENT OF THE MATCH
Daryl Janmaat had only been on the pitch five minutes when he let fly from 25 yards to double Watford's lead.
MOAN OF THE MATCH
The Southampton 'fans' who booed their team off at half-time. Their previously unbeaten team. Who were 1-0 down. At the beginning of September. Come on...
Fraser Forster 6
Cedric Soares 5
Wesley Hoedt 6
Jack Stephens 6
Ryan Bertrand 5
Steven Davis 6
James Ward-Prowse 4
Oriel Romeu 5
Sofiane Boufal 5
Nathan Redmond 6
Manolo Gabbiadini 5
Dusan Tadic (for Ward-Prowse, 46) 6
Shane Long (for Gabbiadini, 69) 6
Charlie Austin (for Boufal, 71) 6
Heurelho Gomes 7
Jose Holebas 7
Christian Kabasele 7
Younes Kaboul 6
Kiko Femenia 6
Abdoulaye Doucoure 8
Tom Cleverley 6
Nathaniel Chalobah 7
Andre Carrillo 7
Andre Gray 7
Adrian Mariappa (for Kaboul, 61) 6
Daryl Janmaat (for Femenia, 61) 7
Troy Deeney (for Gray, 84) 6
WHO'S UP NEXT?
Crystal Palace v Southampton (Premier League, Sept 16)
Watford v Manchester City (Premier League, Sept 16)