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 the same season.
The Hornets also hold the option on an additional 12 months to the deal for the Portugal Under-20 international.
Quina makes six first-team appearances for West Ham and was a key figure in his country’s victory at this year’s Under-19 European Championship.
He becomes the latest arrival at Vicarage Road, as Javi Gracia looks to fill the gap left by Richarlison following the Brazilian's move to Everton to rejoin former boss Marco Silva.
Alongside the signature of former England goalkeeper Ben Foster from West Brom, Watford have also brought in Barcelona forward Gerard Deulofeu and Bologna defender Adam Masina.
The Hornets kick off their new Premier League campaign at home to Brighton and Hove Albion on August 11 before travelling to Burnley eight days later.