The 23-year-old winger has completed his move to the Hornets, becoming the newly promoted side's 10th signing of a busy summer.
Watford has completed the signing of Steven Berghuis from AZ on a four-year deal.
The 23-year-old winger left AZ’s summer training camp in Garderen in order to travel to England for talks with Watford before completing his move to the Premier League newcomer.
Berghuis, who has featured at Under-21 level for the Netherlands national side, will become Watford’s 10th signing following the arrivals of Etienne Capoue, Valon Behrami, Matej Vydra, Sebastian Prodl, Jose Holebas, Jose Manuel Jurado, Giedrius Arlauskis, Allan Nyom and Miguel Britos.