Deeney headed in after just 10 minutes, and after midfielder Mark Yeates had produced a slide-rule finish from a tight angle just before the hour mark, Deeney struck again seven minutes later to take his season's goal tally to eight.
Substitute Matej Vydra rounded off a one-sided second half by converting an 83rd-minute penalty after he had been hacked down from behind by Jimmy McNulty.
On-loan striker Marcus Tudgay did manage to notch a consolation goal for the visitors in stoppage-time.
The win saw the in-form Hornets extend their unbeaten record to seven matches and move into the top six on goal difference, but Barnsley face a winter of discontent, having now gone nine matches without a victory and being dragged ever closer towards the bottom three.
Deeney, recalled to the starting XI, wasted no time in making his mark on the game. He got away from his marker far too easily and stooped to head in Marco Cassetti's excellent 10th-minute corner.
The visitors twice came close to levelling matters from two Akos Buzsaky corners. The first one saw leading scorer Craig Davies produce a superb 15-yard header that Watford skipper Manuel Almunia managed to claw away, then from Buzsaky's second delivery, ex-Arsenal stopper Almunia produced an equally fine save to keep out Tudgay's close-range header.
Sinclair should have brought Barnsley level in the 39th minute when he was sent clear by Buzsaky.
His touch was rather heavy as he rounded Almunia and from a tight angle he saw his shot brilliantly blocked on the line by the covering Tommie Hoban.
While the opening 45 minutes had been very even, Watford dominated the second half - as Barnsley endured a defensive horror show.
Yeates brilliantly found the bottom corner from a tight angle in the 59th minute, before Forestieri teed up fellow forward Deeney for a 16-yard finish in the 66th minute.
And Vydra, having replaced Forestieri, beat goalkeeper Steele from the spot seven minutes from time.
Tudgay notched his second goal in as many matches with a smart finish for Barnsley in stoppage-time, but it was no more than a consolation.