Eden Hazard's fine finish appeared to have saved a point for the Premier League champions, who were again awful after a 3-0 home thumping at the hands of Bournemouth last time out.
However, it only served the sting the Hornets into a late flurry, which handed Javi Gracia a massive win on his home bow as Watford boss.
Tiemoue Bakayoko was booked twice inside five first-half minutes, although the Blues were already in the midst of a sloppy performance, in which they failed to test Watford's back line sufficiently.
And Watford led before half-time when Troy Deeney slammed home a penalty after Thibaut Courtois felled Deulofeu, although the Watford captain could face trouble after celebrating by brandishing his middle finger.
Hazard's impact seemed to have salvaged a draw, but with Chelsea with open at the back, Janmaat restored Watford's lead and Deulofeu, on his home debut, curled home before substitute Pereyra lashed in Watford's fourth to complete a famous win.
Conte spoke pre-match of desiring a statement of support from the Chelsea board, but after three defeats in four games in all competitions his time at Stamford Bridge could be coming to an end, with Tottenham now just a point behind the fourth-placed Blues.
As Chelsea routinely dithered in possession, Watford started on top, with Abdoulaye Doucoure and an unmarked Deeney shooting off-target, and the Blues poor opening to the game was worsened when the struggling Bakayoko was dismissed before the half-hour.
A clumsy foul on Capoue had earned Bakayoko his first booking and his second came soon after, referee Mike Dean brandishing red after a coming-together with Richarlison, although the decision seemed harsh.
Richarlison fired straight at Thibaut Courtois after a one-two with Doucoure, then Dean was at the heart of the action again, awarding Watford a penalty.
Deulofeu poked the ball past Courtois and tumbled over the goalkeeper, with Deeney stepping up to smash home from 12 yards.
Chelsea's lacklustre showing continued after the interval and the lively Deulofeu came close to extending Watford's lead, before David Luiz lost possession to Deeney on the edge of his own box, with Richarlison curling his shot wide.
Conte's response to Chelsea's flat performance was to throw on new signing Olivier Giroud for a debut, and his fellow substitute Cesc Fabregas mustered Chelsea's first shot on target after 80 minutes, having being picked out by the previously anonymous Hazard.
But Hazard suddenly jumped into life, beating Adrian Mariappa and whipping in a delightful 25-yard strike to bring Chelsea level, although within two minutes the champions were behind again.
With Chelsea left short at the back, Janmaat combined with substitute Pereyra in style before beating Courtois, the goalkeeper soon picking the ball out of his net twice more.
Deulofeu's first Watford goal consigned Conte's men to another defeat but worse was to come, Pereyra finding the top corner from a tight angle to wrap up a miserable night for Chelsea.
KEY OPTA STATS
- Chelsea have now lost more league games this season (6) than they did in the entire 2016-17 campaign (5).
- The Blues have lost consecutive Premier League games by a 3+ goal margin for the first time since October 1995 (1-4 vs Man Utd, 0-3 vs Blackburn).
- Watford earned their first win in any competition against Chelsea in 14 attempts (D3 L10), since a 1-0 win in September 1999.
- Javi Gracia is the first Watford manager to win his first home Premier League game in charge of the club.
- Eden Hazard scored his first Premier League goal against Watford; Huddersfield are now the only current side he’s faced but not scored against in the competition.
- All three of Troy Deeney’s Premier League goals this season have been penalties against ‘big six’ sides (Arsenal, Manchester United and Chelsea).
- Three players have now received red cards for fouls on Richarlison this season (Davinson Sanchez, Jonathan Hogg and Tiemoue Bakayoko), more than any other player in the division.