Etienne Capoue scored the only goal to all but secure Watford’s Premier League safety and leave Swansea City still in the bottom three.
Capoue caught Alfie Mawson in possession and raced into the penalty area before scoring at the second attempt.
Watford thought they had doubled the lead when substitute Stefano Okaka fired home from close range, but he was denied for offside.
Gylfi Sigurdsson missed two late chances to equalise, heading just wide before running through and then failing to get enough power behind a shot.
The victory took Watford on to 40 points but left Swansea still two points adrift of safety and without a win in six matches.
See: Watford report | Swansea report