Summer signing Harvey Knibbs scored a stoppage-time winner for Charlton as the Championship new boys made it a losing start for new Watford head coach Paulo Pezzolano.
The 26-year-old attacking midfielder, a transfer window arrival from Reading, pounced from close range in the fourth minute of time added on.
Charlton, last season's League One play-off winners, came out of the blocks quickly, with Hornets goalkeeper Egil Selvik making a fine reaction save to deny centre-back Lloyd Jones from converting Rob Apter's cross.
Watford settled down and looked more fluid in possession but the Addicks produced the better opportunities in the opening 45 minutes.
Right wing-back Apter, a summer signing from Blackpool, produced a sharp turn and strike from the edge of the box which went narrowly past the left-hand upright in the 26th minute.
The opportunity arose after the persistence of Sonny Carey, also snapped up from the Tangerines on a free transfer as one of nine new arrivals, won the ball in the Watford penalty area.
The Hornets' threat largely came from powerful and tricky winger Kwadwo Baah. It was Baah's 27th-minute shot which fell nicely for striker Luca Kjerrumgaard but the Udinese loanee's low attempt was blocked by the legs of Jones from 10 yards.
Jones proved to be dominant at both ends of the pitch for Charlton and once again forced Selvik into action in the 40th minute. This time the Norwegian plunged to his right to push away Jones' header from Apter's free-kick.
Charlton fashioned another opportunity just before the hour but Charlie Kelman's deft flick on Tyreece Campbell's near-post shot was kept out by Selvik, who also smothered Carey's strike from a tight angle.
Watford midfielder Hector Kyprianou guided a header harmlessly wide, a waste after being found unmarked in the penalty box from Jeremy Ngakia's cross in the 68th minute.
Knibbs was brought on in the 80th minute for Campbell and did not take long to start paying back a chunk of his transfer fee.
Conor Coventry sent a corner deep to the back post which Jones helped back into the danger zone and the debutant clinically lashed the ball into the net with a nerveless first-time finish.
Watford produced some dangerous crosses but struggled to force home goalkeeper Thomas Kaminski into serious action.
Charlton are unbeaten on their own patch since December 3 and have won 14 and drawn three of their last 17 matches, including the play-off semi-final second leg.