Despite spending one season on loan with lower division side AC Giacomense as a teenager, Masina has spent the entirety of career at the Stadio Renato Dall'Ara.
The 24-year-old has made over 130 appearances for Bologna throughout his career, scoring four goals and claiming seven assists. Masina is also just one game short of reaching a century of appearances in the top flight of Italian football.
Masina has previously been linked with a move to Tottenham, although the proposed deal never came to fruition. But the Bologna defender will now have the chance to move to London after his club agreed to a deal with Watford ahead of the 2018/19 campaign.