Lee Hughes, West Brom's skipper for the day, grabbed his 21st goal of the season to condemn Watford to a heavy 3-0 defeat at the Hawthorns.
Hughes struck in the 57th minute after a shocking blunder by Watford keeper Espen Baardsen.
The big Norwegian fumbled a ball from Albion defender Neil Clement and Hughes pounced to make it 2-0 with a header into the back of the empty Watford net.
Albion, who reinforced their play-off challenge with this victory, had to wait until five minutes after the break for their breakthrough.
Des Lyttle slipped a perfect pass to transfer listed Irish striker James Quinn, who came on as a substitute at half time, and he grabbed his first league goal in two years with an angled drive into the far corner of the net.
The scoreline was completed in injury time when Albion's man of the match Clement found the net with a freekick from 25 yards that took a deflection before flying past the helpless Baardsen.
It was Watford, now with just one win in their last nine games, who were the sharper outfit in the opening 45 minutes and they squandered a greatopportunity of taking an early lead.
A mistake by defender Phil Gilchrist opened a path to goal for striker Tommy Smith but, from only 12 yards, he ballooned his shot high over the crossbar.
After the break Watford were again guilty of poor marksmanship and in the dying seconds Allan Smart missed an open goal after Gifton Noel-Williams won a challenge with Albion 'keeper Russell Hoult.
Teams
West Brom: Hoult, Lyttle, Clement, Butler, Cummings (Quinn 45),
Jordao, Appleton, Sneekes, Taylor (Roberts 75), Hughes,
Gilchrist.
Subs Not Used: Jensen, Sigurdsson, Balis.
Booked: Appleton.
Goals: Quinn 52, Hughes 59, Clement 90.
Watford: Baardsen, Cox, Robinson, Page, Palmer, Hyde, Vernazza,
Nielsen, Helguson (Noel-Williams 60), Wooter (Kennedy 80),
Smith (Smart 60).
Subs Not Used: Chamberlain, Panayi.
Booked: Robinson.
Att: 17,261
Ref: M Pike (Barrow-in-Furness).