"I've been after him for a long time," manager Aidy Boothroyd told the club's official website.
"He's two footed, he can shoot, he can cross, he can make goals and he can score goals.
"I think he's still got a lot more to come but he's athletic, he fits the bill and he is hungry enough to want to play in the Premiership.
"He's got a little bit more experience for us. He's knows the division and he knows what's required to win.
"Jobi will stamp his own personality on Watford and become a fans' favourite very quickly."