Goals from Cristian Battocchio and Troy Deeney put the Hornets firmly in command inside the opening 20 minutes and they largely eased through the rest of the contest before substitute Alexander Merkel added a third with 16 minutes remaining.
Beppe Sannino's men came into the game on the back of five successive home wins in all competitions and it did not take them long to be on course for a sixth.
A good ball out to Daniel Pudil on the left flank set the fifth-minute attack again and although the Czech's cross was behind Deeney and Ikechi Anya, the ball fell for Battocchio 20 yards out and he had the time to take a touch before hitting a sweetly-struck half-volley past Luke Steele to make it 1-0.
Davide Faraoni was twice denied by the Tykes keeper, the first an instinctive reaction stop, before the Hornets doubled their advantage in the 16th minute.
Again the left side of the penalty area proved a productive route for the hosts as they built nicely and although Lewis McGugan mis-hit a shot, the ball broke nicely for top scorer Deeney to fire in his 17th of the campaign.
Having established a firm early grip on the contest, the Hornets were comfortable for the remainder of the first half without seriously threatening to extend their lead, although they still sought to push forward when the opportunities presented themselves.
Watford maintained their ascendancy after the break and Tykes boss Danny Wilson decided he had seen enough just before the hour-mark and made a double substitution.
But within moments of Chris O'Grady and Reuben Noble-Lazarus entering the fray, Watford spurned a fine opportunity to add a third as the unmarked Daniel Tozser headed an Anya cross from the left down and wide of the target.
The 65th minute of the match saw Watford pass the ten-hour mark since they last conceded a goal at Vicarage Road and nine minutes later they added a third.
Stand-in striker Anya was the creator this time with some fine close skill on the left side of the penalty area before he pulled the ball back to substitute Merkel, who made no mistake from the edge of the six-yard area to net his first goal for the Hornets.
Barnsley almost scored a scarcely-deserved consolation with four minutes remaining when Nick Proschwitz got on the end of a ball forward and lobbed it goalwards with an outstretched right leg, only to see it rebound off the crossbar before it was cleared behind. From the resultant corner, Jean-Yves M'Voto sent a header wide.