Deeneys goal is the difference as the Hornets win at home again.
Deeney beat the offside trap from Will Hughes excellent through-ball and fired high beyond Ben Foster with 13 minutes to go to increase the Hornets top-half credentials.
West Brom had chances to take the lead and might have got something from the game had Grzegorz Krychowiak hit the target early on, or Salomon Rondon done better when one-on-one after the break.
But Deeneys strike condemned West Brom to a sixth straight defeat and put them eight points from Premier League safety, while the Hornets move a point within eighth-placed Leicester.
The Baggies shaded the opening half, and Krychowiak tested the hosts nervous looking-defence with a powerful drive inside four minutes, which may have put them ahead but for Adrian Mariappas crucial block.
West Brom came to frustrate Watford and had done a real job on the hosts until one mix-up shortly before the break allowed Daryl Janmaat a clear route to goal, only for his finish to make life easy for Ben Foster who turned it behind.
The half-time team talks helped to liven up a limp encounter and Orestis Karnezis saved well at his near post from Rondons 20-yard shot minutes after the restart.
Chris Brunt was kept in the West Brom starting line-u despite his dressing room dissent of Alan Pardew, while Gareth Barry missed out with a knee injury.
But the key moments again evaded Pardews side, who were inches from scoring when Jonny Evans header was tipped onto the post by Karnezis.
And soon after they were behind when Hughes spotted Deeney in space and with Evans trying to step up, the Hornets captain raced through before finishing with real confidence into the top of the net.
Pardew threw on Oliver Burke and Sam Field in the dying moments in a desperate attempt to draw level, but will be increasingly concerned about his own prospects, as well as West Broms, following a run of eight points from 15 Premier League games.
