Hameur Bouazza has joined Fulham for a fee of up to £4m after agreeing terms with the Cottagers.
The Algerian international has signed a three year deal with Fulham and will now link up with Laurie Sanchez's side after a four year stint with the Hornets.
Speaking of Bouazza's move Hornets' manager Aidy Boothroyd said: "We're disappointed to lose him because I wanted to keep him in the squad for this season.
"Sometimes though you have to weigh up the money on offer and the value a player brings to the squad who wouldn't necessarily be starting each and every week.
"He is definitely a terrific player in my eyes, but has got an awful long way to go to be all he can be in his career. Sometimes as a player people get in your ear, and it takes a very strong person to say 'I'm not ready to move yet'."