Ogbonna, who made an appearance from the bench in West Ham’s Europa Conference League final victory in 2023, is Watford’s eighth summer signing.
He made over 200 appearances for the Hammers in the Premier League, with ex-Watford boss Gianluca Zola the only Italian to have exceeded that figure.
“It is very important to bring in some experience, like with Moussa [Sissoko],” Ogbonna told the club website.
“I have spoken with the manager a couple of times, so I know his style of play. I am here to try my best and to try to help the team.”