Spurs started with for changes from the win over Middlesbrough, with Toby Alderweireld preferred to Davidson Sanchez. Serge Aurier, Japhet Tanganga and Dele Alli were the other changes.
A penalty appeal for Spurs early on against Craig Cathcart fell on deaf ears, as Heung-min Son turned on the inside left channel for a strike at Ben Foster, who gathered easily.
After weathering the early storm from Spurs, the Hornets grew into the match later in the half forcing Paulo Gazzaniga to a parry Gerard Deulofeu's cross-cum shot.
Spurs still managed to put up a fight seven minutes to the break when Lucas Moura ran onto a pass from Alli, but Foster was quickly off his line to avert the danger.
Having won only one of their last13 league matches against Spurs, the Hornets were out firing in the second half as Olivier Doucoure flashed into the side-netting from good work by Ismaila Sarr.
Spurs, who were on the end of a 1-0 loss to title-chasing Liverpool last weekend, went close to opening the deadlock on 61 minutes but Erik Lamela failed to finish of Son's assist.
The Hornets had a golden chance to go ahead on 70 minutes after Jan Vertonghen handled Deulofeu's shot in the area to give away a penalty, which Troy Deeney failed to bury.
Want-away attacker Christian Eriksen came on for Alli three minutes later, with new signing Gedson Fernandes given his debut by Jose Mourinho, as Spurs looked for a late winner.
Honours even it ended, as Spurs for the second time in a week fail to close in on the top four.