Earlier this year, Deeney was twice on the mark in a 4-0 win against the Blues and on Saturday he scored a vital early winner against his boyhood club to give the Hornets a perfect start to the new campaign.
The confidence he displayed in notching his first-half goal highlighted why he is one of the most dangerous strikers in the division.
The visitors made a positive start to the new campaign as if to make amends for their disappointment last May.
Birmingham, on the other hand, were a little disjointed in their 1,000th home game in the second tier of league football although they displayed a lot of spirit in the second half when they were unsuccessful in their bid to snatch a point.
Following an early Birmingham onslaught it was Deeney who demonstrated the kind of clinical finishing which made him so outstanding last season.
Birmingham attempted to over-run the Hornets but it was Watford's confidence on the ball which emerged as a feature of the game.
This was shown when Fernando Forestieri played a quick one-two with Deeney, which resulted in the Watford striker guiding home a low right-foot shot to the left of Darren Randolph.
With eight newcomers, none of whom cost a fee, in the ranks, the hosts had to regroup as quickly as possible but it was Watford who slipped effortlessly into the groove to display a high class of football.
Ikechi Anya proved a live-wire on Watford's left and gave Birmingham's big defence a torrid time when in possession.
There was, however, plenty of promising play from Birmingham with the former Inverness midfield man, Andrew Shinnie, in particular showing positive approach in his first ever Championship game.
As Watford gained the upper hand in the closing stages of the first half it was significant that Birmingham had three players, Tom Adeyemi, Dan Burn and skipper Paul Robinson booked for reckless tackles.
Birmingham produced a battling second-half performance but the lack of fire-power against a resolute Watford defence.
Their best goal attempts came from a Wade Elliott effort which clipped the cross bar and a bobbling shot by Burn which goalkeeper Manuel Almunia hastily pushed clear.
Burn, on loan from Fulham, continued to be Birmingham's main threat.
The 6ft 6ins tall central defender moved up for a corner only to see his header fly just wide in a hectic finish which saw Watford hanging on grimly to their slender lead.
Tempers became a little frayed and Lee Novak became the fourth Birmingham player to receive a yellow card as the hosts grew frustrated by their inability to obtain a face saving equaliser.