After a desperately poor first half, the game sparked into life in the 53rd minute as Abdoulaye Doucoure scored a comical own goal to hand the visitor the lead.
That setback only served to motivate Javi Gracia's side, which stormed back with two headed goals from Troy Deeney and Craig Cathcart in the space of seven minutes to join Liverpool and Chelsea on maximum points from the opening four games.
It was a result that brings Spurs, who had won their opening three matches, crashing back down to earth after their 3-0 win over Manchester United.
During a tepid opening both sides exchanged headed chances, Dele Alli guiding his wide of Ben Foster's post and Deeney powering one over the crossbar from just six yards out.
Tottenham looked the most threatening for the remainder of the half, but its only real chance came a minute before the interval when Davinson Sanchez headed over Jan Vertonghen's teasing cross from the left.
Christian Eriksen signalled Tottenham's intent at the start of the second period, firing in long-range drives that were held by Foster, before Doucoure's slapstick intervention handed Spurs the lead.
Ben Foster looked primed to collect a looping ball before Christian Kabasele headed it almost out of his hands and into the path of Lucas Moura, whose delivery back across goal was chested into his own net by the hapless Doucoure.
This woke Watford from their slumber and they almost levelled after 67 minutes when Toby Alderweireld headed against his own crossbar before Deeney nodded in from Jose Holebas' fine free-kick shortly after to level things.
Holebas turned provider again for the winner in the 76th minute, his outswinging corner wonderfully headed into the top corner by Cathcart to continue his side's remarkable start to the season.