Super Eagles striker Emmanuel Dennis ended his seven-game barren run after he scored the equalizer for Watford, but their survival hopes were crushed as a late goal from Jansson ensured they lost 2-1 at Vicarage Road.
The Bees traveled to Vicarage Road on the back of two brilliant wins against high-flying Londoners Chelsea and West Ham. But Watford was the opposite, as they had won just one of their last seven games.
The game was balanced from the start, but it was Brentford who drew first blood through Christian Norgaard, who scored a tap-in after receiving a pass from Kristoffer Ajer.
They saw out the first half with their one-goal lead despite advances from Watford.
Watford was more offensive in the second half, and within nine minutes from the break, they restored parity. Dennis fired in a rebound to score his first goal in eight games.
Watford had chances to take the lead, but they were profligate in front of goal. Imran Louza could have finished the game off in stoppage time, but he fluffed his lines.
However, the Hornets were punished with seconds left in the game. Pontus Jansson leaped highest to a Christian Eriksen free-kick to head in behind Ben Foster to secure a 2-1 victory for the Bees.
The loss puts paid to Watford's chances of survival, as they sit 19th, six points away from safety with six games left.
