Leading the line in our 4-2-3-1 system, Defoe was in the right place at the right time to head home Kyle Walker's cross in the 55th minute of the match to mark Lloyd Doyley's Testimonial.
That was the difference in a game of few opportunities.
We had the better of the chances, epecially in the second half, while ex-Spur Mark Yeates went close twice for the home side.
There was no little effort but only two chances in the first half.
The first fell to Yeates, who looked to whip the ball in from the right but his effort looked on target and Brad Freidel stretched to his right to tip away.
At the other end, William Gallas rose high to connect with Tom Carroll's inswinging corner but planted his header a yard over.
We got to the byline on a few occasions without reward and looked to get midfielders breaking from deep while out of possession, the front four pressed high up the pitch to win the ball back.
Carlo Cudicini, Kyle Walker, Jan Vertonghen and Gylfi Sigurdsson all entered the action for the second half and Vertonghen's ability to bring the ball out of defence certainly caught the eye as we pushed on.
The Belgian defender set-up the first chance of the half with a 50-yard stride forward and pass to Gareth Bale, who found Doyley in the way of his goalbound strike.
The breakthrough wasn't far away though. Walker controlled after Sigurdsson switched play to the right and darted to the byline before clipping in the perfect cross for Defoe, unmarked, to power his header past Manuel Almunia.
Watford's best chance followed just after the hour as Yeates latched onto Carlo Cudicini's clearance, got into the area and curled inches wide of the far post.
Harry Kane, on for Defoe on 66 minutes, lobbed a half-chance wide and then Bale started and finished a move, switching play again to Walker on the right and then getting on the end of the cross, a strong downward header unfortunately straight at Almunia.
Andros Townsend came on for Bale with 15 minutes remaining and provided the final effort of note, a stinging half-volley from the edge of the box that Almunia did well to touch wide.
Watford (4-1-4-1): Almunia; Doyley (Hodson, 75), Nosworthy, Taylor, Pudil (Dickinson, 7); Eustace; Yeates, Hogg (Forsyth, 66), Buaben (Massey, 78), Murray (Mingoia, 84); Garner (Iwelumo, 58).
Spurs (4-2-3-1): Friedel (Cudicini, 46); Naughton (Walker, 46), Kaboul, Gallas (Vertonghen, 46), Assou-Ekotto; Jenas (Sigurdsson, 46), Livermore (Bentley, 79); Carroll (Huddlestone, 66), Lennon, Bale (Townsend, 75); Defoe (Kane, 66). Unused subs: Bassong, Dawson.
Goal: Spurs - Defoe (55).
Referee: Mr T Kettle.