Report: Swans 1 Watford 2
Richarlison's 90th-minute strike condemned the Swans to a third consecutive home defeat at the hands of Watford.
The Swans had been looking on course for a first point of the season at the Liberty, only for the Brazilian to take advantage of a defensive mishap and fire home the winner via the crossbar.
Watford, who had been on top for the first half, deservedly led at the break via Andre Gray, who slammed home after his cross dropped back to him via Sam Clucas' touch.
The Hornets went close to doubling their lead just past the half-hour mark only for Andre Carillo to be ruled offside.
A weak Alfie Mawson passback then allowed Gray another chance to double the lead, only for the striker to rifle his effort.
The Swans made a double change at the break with Roque Mesa and Tammy Abraham on for Mike van der Hoorn and Sam Clucas.
It looked to have paid off when Wilfried Bony's shot was parried by Heurelho Gomes and Abraham tapped home from close range.
But the warning signs were there when sub Troy Deeney almost won it for Watford in the closing stages, only for Lukasz Fabianski to deny him low down.
Watford, though, would find their winner in stoppage time as Roque Mesa's pass headed to Richarlison, who got the better of Mawson in a 50-50, and he struck home from 20 yards with his shot rebounding in off the crossbar.
The defeat left the Swans on five points from six games, with nothing to show from their first three home games.
Swans: Lukasz Fabianski, Mike van der Hoorn (Tammy Abraham, ht), Federico Fernandez (capt), Alfie Mawson, Kyle Naughton, Leroy Fer, Sam Clucas (Roque Mesa, ht), Tom Carroll, Martin Olsson, Jordan Ayew, Wilfried Bony (Renato Sanches, 85).
Subs: Kristoffer Nordfeldt, Angel Rangel, Wayne Routledge, Luciano Narsingh.
Watford: Heurelho Gomes (capt), Adrian Mariappa, Christian Kabasele, Jose Holebas, Kiko Femenia, Tom Cleverley, Abdoulaye Doucoure, Etienne Capoue (Molla Wague, ht), Andre Carillo (Roberto Pereyra, 74), Richarlison, Andre Gray (Troy Deeney, 85).
Subs: Orestis Karnezis, Daryl Janmaat, Isaac Success, Ben Watson.