Double blasts by Don Hutchison and Marlon Harewood stung the Hornets as West Ham all but booked their play-off place with an energetic and emphatic victory at Upton Park.
"We're within touching distance now," smiled Alan Pardew, calculator in hand, knowing that his side only need to avoid a four-goal defeat at Wigan Athletic next weekend.
"I thought we played very, very well and with three successive wins and four clean sheets were showing the right statistics needed for promotion." In this, their penultimate game of the campaign, Hammers knew only maximum points would do from their final match at Upton Park if they were they to cement a top-six spot.
But Watford's cynical early spoiling tactics looked like frustrating the Eastenders as Bobby Zamora was unceremoniously chopped by the subsequently booked Paul Mayo.
When Steve Lomas was similarly upended by Micah Hyde on the edge of the penalty area on 17 minutes, however, Watford paid the price as Hutchison, making his first start since January 10, drilled the resultant free-kick wide of the defensive wall, beyond Alec Chamberlain and into the bottom left-hand corner of the net.
In reply, Bruce Dyer's angled screamer was tipped over by Stephen Bywater before the back-pedalling Hammers' keeper palmed away Lee Cook's awkward touchline free-kick midway through the half.
Buoyed by the cushion of their lead, though, West Ham were well on top although the creativity of Matthew Etherington and Michael Carrick was matched by the wastefulness of Harewood and Zamora in front of goal.
On 44 minutes, though, Hutchison again showed his strikers the way to goal when he glanced Christian Dailly's goalbound header over the line after Etherington delivered a well-flighted corner into the heart of the Watford area.
And West Ham carried on from where they left off after the break, too, as Chamberlain formed a one-man barrier to thwart Etherington, Harewood and the workaholic Lomas.
Such was the Hammers' urgency that manager Pardew was involved in their third goal when he hurled the ball to Lomas, whose quick throw-in duly released Zamora who was bundled over by Mayo 12 yards out.
It was hardly any surprise when Chamberlain brilliantly beat away Harewood's subsequent spot-kick, but having spotted an infringement, referee Jones ordered the penalty to be retaken and this time the Hammers striker made no mistake with a clinical shot.
Still the Watford keeper was the busiest man of the park as he foiled Carrick and then Zamora with two fine saves, while also rushing out to clear the ball into the face of an unfortunate, female front-row fan whose pain would have been eased by the Harewood's stoppage-time tap-in.
It was his 25th goal of the season, after runaway substitute Jobi McAnuff's low shot was parried into his path by the unlucky Chamberlain.
"West Ham had quality throughout their side, they were really fired up and were a real goal threat," admitted Watford boss Ray Lewington.
"They must have a great chance in the play-offs because they've got the momentum with them.
"As far as I'm concerned, we're already safe from relegation and that is about as positive as I can be."