The win put fourth-placed Spurs five points clear of Chelsea with three games remaining and within a point of Liverpool, who have two matches left, in the race for Champions League spots.
Tottenham took the lead after 16 minutes when Watford keeper Orestis Karnezis spilled a cross from Kieran Trippier and Christian Eriksen scooped the ball to Alli who had the simple task of stroking the ball home from near the penalty spot.
Kane put Spurs two goals ahead in the 48th when he tucked away a cross from the right by Trippier and they almost had a third when Jan Vertonghen hit the post with a late header.
Tottenham have 71 points ahead of London rivals Chelsea on 66, both from 35 games, while third-placed Liverpool have 72 from 36. Liverpool play Chelsea at Stamford Bridge on Sunday.
