Despite losing top scorer and influential midfielder Tom Cleverley to a serious-looking injury and having Jon Harley sent off, the Hornets looked set to secure a vital three points when Danny Graham scored a superb 25-yard goal with five minutes remaining.
But the Baggies, who had not registered an attempt on target in the 90 minutes, kept going and rewarded very late on when Andwele Slory's header back across the area found Brunt unmarked at the back post to level.
Despite the immense disappointment, the good news for the Hornets was that they remained outside the drop zone on goal difference from Sheffield Wednesday, while the Baggies' promotion party remains on hold for one more game at least.
Watford spurned a great chance to take an early lead in the eighth minute when John Eustace nodded the ball through to Adrian Mariappa, who found himself all alone in the area.
The defender had time to bring it down but chose to hit it first time on the half-volley and put his shot too close to Scott Carson.
The home side, who lost defender Lloyd Doyley to injury early on, had the better of it thereafter in the first half, although the match sparked into life when Eustace, Graham Dorrans and Roman Bednar were both booked following a touchline scuffle.
A Mariappa shot at Carson aside, the second half was a tail of injuries for both sides which culminated in Cleverley losing out on a challenge to Jonas Olsson that left him lying on the ground.
The Manchester United loanee had to be stretchered form the pitch wearing an oxygen mask.
If that wasn't bad enough, the home side were reduced to ten men 10 minutes later when Harley was adjudged to have blocked off James Morrison and referee Steve Tanner had no hesitation in producing a second yellow.
However, with five minutes remaining, Don Cowie won the ball and laid it off to Graham, who struck a superb first-time right-footed half-volley from 25 yards that gave Carson no chance.
Watford looked like they would hold on for their most important three points of the season after Morrison had incredibly missed a free header from three yards out.
But in the last minute of stoppage time, Slory headed across the area and Brunt came in unmarked to head in the equaliser.