Match ends, Middlesbrough 0, Watford 1.
Jose Holebas' fine strike settled a poor game as Watford's victory left Middlesbrough still looking for a first home win of the Premier League season.
The Greece defender drove past Victor Valdes from 20 yards to provide some inspiration for a forgettable match generally devoid of quality.
Boro - who remain above the bottom three on goal difference only - failed to properly test Hornets goalkeeper Heurelho Gomes at all during the game.
The result moves Watford up to 10th.
Middlesbrough have now lost three home games in a row for the first time in seven years - and have not won in the top flight at the Riverside since April 2009.
- Relive the action from the Riverside and Southampton v Burnley
- Football Daily: 'Comfortable' Saints continue unbeaten run
No home comforts for Boro
Middlesbrough won promotion back to the Premier League after seven seasons on the back of a formidable defence and strong home record - which they have so far failed to bring to the top flight.
Aitor Karanka's side conceded just 31 times last season - eight at home - but have already let in six at the Riverside in a run which has seen them take just a point in front of their own fans.
But while they still looked untroubled by Watford for large periods, it will be their lack of attacking threat that should most concern Karanka.
They managed just three tame efforts on target, Adam Forshaw wasting their best opening with little more than a backpass from the edge of the area.
Striker Alvaro Negredo - who scored 23 times in his first 33 appearances for Manchester City, and then none in his final 16 - now has just a goal on the opening day to his name in Boro colours and never looked like adding to it.
And with trips to Arsenal, Manchester City and Leicester in their next three away games, Boro must find goals from somewhere if they are to compete.
Success sparks for Watford
Watford captain Troy Deeney had complained that his players were "bullied to a man all over the pitch" in the 2-0 loss at Burnley in their previous away match, but the Hornets rarely looked troubled by a toothless Boro side.
Walter Mazzarri deployed Deeney as a lone striker, while record signing Isaac Success made his first start for the club wide on the left - and the Nigerian was responsible for most of the game's sparse moments of promise.
The match may have opened up had Middlesbrough right-back Antonio Barragan seen a second yellow card on the half-hour mark for tripping Success, but referee Roger East gave the defender a reprieve.
And Success was involved in the build-up to the winning goal, going down after a typically direct run into the area. While he protested and claimed for a penalty, Holebas nipped in front of Cristhian Stuani to control a poor clearance, take a touch and then fire a swerving shot that former Barcelona keeper Valdes could only help into the top corner.
It was Watford's first shot on target but enough to give them the points.
Man of the match - Isaac Success (Watford)
The stats you need to know
- Watford have two victories in their past three away Premier League games, having won just two of their previous 12.
- No Premier League defender has had a hand in more goals this season than Jose Holebas (three - two goals and one assist).
- Boro have won just two of their past 18 home Premier League games (D10 L6).
BBC Radio 5 live pundit Chris Sutton:
"Middlesbrough were pitiful going forward. Well - there wasn't any forward play.
"It looks like it's going to be a tough season for them. They are going to have to start winning at home. I have to say Alvaro Negredo didn't look like he wanted to be on the pitch."
BBC Radio 5 live commentator Alan Green:
"I actually haven't seen anyone worse than Middlesbrough this season."
What the managers said
Middlesbrough manager Aitor Karanka:
"When they scored it was impossible to play. Watford have really experienced players. We need to learn and keep going.
"The only way we know how to play is the way we played the first 35 minutes."
Watford boss Walter Mazzarri:
"I'm very satisfied because we won an away game in such a difficult stadium. I'm mainly (satisfied) with the character of the team but I'm less satisfied on the overall performance.
"I'm happy. It's going very well. We could've had one or two points more at this stage. We've been playing all the games well with the exception of Burnley away."
Middlesbrough travel to second-placed Arsenal next Saturday (15:00 BST kick-off), while Watford take on Swansea at the Liberty Stadium at the same time.
- 17BarragánBooked at 21minsSubstituted forAyalaat 69'minutes
- 14de RoonBooked at 74minsSubstituted forRhodesat 88'minutes
- 18StuaniBooked at 38minsSubstituted forTraoréat 69'minutes
- 21RamírezBooked at 42mins
- 7N AmrabatBooked at 65minsSubstituted forZuñigaat 72'minutes
- 3BritosSubstituted forKabaseleat 85'minutes
- 10SuccessSubstituted forWatsonat 89'minutes
- Roger East
- Shots on Target
Second Half ends, Middlesbrough 0, Watford 1.
Foul by George Friend (Middlesbrough).
Roberto Pereyra (Watford) wins a free kick on the right wing.
Hand ball by Etienne Capoue (Watford).
Corner, Watford. Conceded by Calum Chambers.
Gastón Ramírez (Middlesbrough) wins a free kick in the defensive half.
Foul by José Holebas (Watford).
Foul by Stewart Downing (Middlesbrough).
José Holebas (Watford) wins a free kick on the left wing.
Substitution, Watford. Ben Watson replaces Isaac Success.
Substitution, Middlesbrough. Jordan Rhodes replaces Marten de Roon.
Corner, Middlesbrough. Conceded by Juan Zuñiga.
Substitution, Watford. Christian Kabasele replaces Miguel Britos because of an injury.
Foul by Marten de Roon (Middlesbrough).
Isaac Success (Watford) wins a free kick in the attacking half.
Delay over. They are ready to continue.
Delay in match Miguel Britos (Watford) because of an injury.
Attempt missed. Marten de Roon (Middlesbrough) header from the centre of the box misses to the left. Assisted by Stewart Downing with a cross following a corner.
Corner, Middlesbrough. Conceded by Juan Zuñiga.