Italian international Diego Fabbrini, who made his debut for the Azzurri in a friendly against England last August, has joined the club on a four-year-deal alongside defender Gabriele Angella.
Last year's loanee midfielders Almen Abdi and Cristian Battocchio have returned to Vicarage Road on permanent contracts with both being tied down for another three years.
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"The club has received confirmation from the Football League and The Football Association that the contracts of Almen Abdi, Cristian Battocchio, Gabriele Angella and Diego Fabbrini have been registered."
The Hornets have also secured the transfers Marco Cassetti and Daniel Pudil - from second affiliate club Granada - who both return to the club having initially enjoyed successful loan spells last season.
Javier Acuna and Davide Faroni will also join the club on permanent deals, however, as the club remains under a transfer embargo they must await ratification from the Football League for the signings of the latter four.
The statement continued: "The club anticipates receiving the same confirmation of their contract registration from the football authorities shortly."