Long-range strikes from Abdoulaye Doucoure and Daryl Janmaat ended Southampton's unbeaten start and lifted Watford into the top four.
The goals 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.
Meanwhile, shot-shy Southampton drew a blank for the third time in their four Premier League matches so far this season.
They have issues to address at the back, too, as there was again no Virgil van Dijk in the line-up.
Southampton 0-2 Watford
