The 18-year-old joins the Hornets on a contract that will take him through to 2022, with the option of a further year.
Watford have completed the signing of West Ham midfielder Domingos Quina on a four-year contract.
The 18-year-old, who spent time in Chelsea’s youth system, made his professional debut in the 2016-17 season for the Hammers, and featured twice in their Europa League campaign of the same campaign.
The Hornets also hold the option on an additional 12 months for the Portugal Under 20 international.