Spurs are the only team with a 100 per cent record.
Son’s first-half free-kick was the difference between the two sides in north London and means that Nuno Espirito Santo becomes the first Spurs boss to win his first three league games in charge since Arthur Rowe in 1949.
All three have been by a 1-0 scoreline following narrow victories over Manchester City and Wolves and – being the only team with a 100 per cent record from their first three games – there is early-season optimism that Nuno’s side might be able to challenge for the top four.
They will face sterner tests than this as although Watford were competitive, the hosts were always in control.
Moussa Sissoko was named in the Watford side, less than 48 hours after his £3million switch from Spurs, who were surprised by his inclusion.
Watford registered the Frenchman before Friday’s 12pm deadline and the deal was concluded in order to make him available.
And Spurs were almost made to instantly regret not delaying the deal as Sissoko nearly set up a first-minute goal for his new team.
After Spurs had a penalty claim rejected, the Hornets broke at speed down the left through the dangerous Emmanuel Dennis and as the ball fell to Sissoko at the far post he teed up Juraj Kucka, whose goalbound shot was blocked by the head of Eric Dier.
Unlike their opening day win against Manchester City, the onus was Spurs to attack and create chances.
A great move almost resulted in the opening goal as Japhet Tanganga’s pull-back on the byline was met by Steven Bergwijn and his shot was well blocked by William Ekong.
Tanganga was involved again from a set-piece as he shovelled a loose ball towards goal, but Daniel Bachmann saved with his feet.
The breakthrough came in the 42nd minute as Son marked his landmark appearance with a goal.
He did not mean it, though, as his in-swinging free-kick from the left evaded everyone, was misjudged by Bachmann and found its way into the far corner.
Sissoko again threatened to do some damage as Watford started the second half well, powering to the byline and getting a cross in for Ismaila Sarr, whose shot was blocked.
With the Hornets having to offer more going forward in search of a leveller, it opened things up for the hosts on the counter attack and they were inches from getting a second in the 67th minute.
Son fed Harry Kane and the England captain picked out Dele Alli with a perfectly weighted pass, but Alli’s first-time shot drifted just wide of the post.
Spurs began probing for a second and substitute Lucas Moura forced Bachmann into a low stop with a shot across goal and then the Austrian goalkeeper had to scramble to palm away Pierre-Emerick Hojbjerg’s deflected free-kick.
Kane had two quickfire chances to kill the game in the final 10 minutes as he could not connect with Moura’s cross at the far post and then had a shot saved well by Bachmann from the resulting corner.