Deeney found the net 20 times last season and he already has four strikes to his name this term after helping Gianfranco Zola's side put promoted Bournemouth to the sword at Vicarage Road.
Gabriele Angella, a summer signing from Udinese, gave Watford a 13th-minute lead and then restored the hosts' advantage early in the second half after Lewis Grabban equalised for Bournemouth.
Deeney opened his account with a deflected shot in the 56th minute and after Lewis McGugan, another new signing, made it 4-1, Deeney slammed home Watford's fifth goal before completing his treble with a 90th-minute penalty.
Watford are top of the table on goal difference from Blackpool and Nottingham Forest, both of whom claimed 1-0 victories with dramatic late goals.
Blackpool prevailed thanks to an injury-time own goal from Barnsley's Scott Wiseman, while Darius Henderson exploited a stoppage-time error by Blackburn Rovers goalkeeper Jake Kean to earn Forest a first league win at Ewood Park since 1999.
Queens Park Rangers, favourites for promotion following their relegation from the Premier League last season, were held to a 1-1 draw at Huddersfield Town.
Former Everton striker James Vaughan gave the Terriers a 35th-minute lead from an Oliver Norwood cross, but David Hoilett levelled just three minutes later for Harry Redknapp's visitors.
New signing Charlie Austin came close to snatching victory for QPR shortly before the hour, but his header came back off the crossbar.
Brighton and Hove Albion saw a forgettable week come to an end with a 2-1 defeat at home to Derby County.
Losing semi-finalists in last season's play-offs, Brighton were dumped out of the League Cup by fourth-tier Newport County in mid-week and were beaten again after Chris Martin replied to Jose Ulloa's opener with a brace.
Like Brighton, Millwall remain without a point to their name following a 3-0 defeat at Ipswich Town, while Charlton Athletic have also lost twice after Lukas Jutkiewicz scored the only goal in a 1-0 win for visiting Middlesbrough.
Yeovil Town's first ever home game in the second tier ended in disappointment as they lost 1-0 to Birmingham City, but there was better news for Burnley, who won 2-1 at Sheffield Wednesday.
In the day's other fixture, Nick Blackman earned Reading a 1-1 draw at Bolton Wanderers with a 51st-minute penalty after the hosts had taken an early lead through a Darren Pratley header.
Leicester City and Leeds United meet on Sunday, both sides having won their opening games, while FA Cup holders Wigan Athletic are not in league action due to their Community Shield date with Manchester United.
Watford smash six in Championship, QPR held
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